This is difference between the default preference of Firefox 28.0 beta 8 and one of 27.0.1.

Default pref Javascript files (Firefox 28.0 for Linux x86_64):
(omni.ja){/greprefs.js,/defaults/pref/services-sync.js}, (browser/omni.ja)/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js}, (webapprt/omni.ja)/defaults/preferences/prefs.js
Default pref Javascript files (Firefox 29.0 beta 3 for Linux x86_64):
(omni.ja){/greprefs.js,/defaults/pref/services-sync.js}, (browser/omni.ja)/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js}, (webapprt/omni.ja)/defaults/preferences/prefs.js
PrefFirefox 28.0Firefox 29.0 beta 3
browser.bookmarks.max_backups1015
browser.customizemode.tip0.learnMoreUrlundefinedhttps://support.mozilla.org/1/firefox/%VERSION%/%OS%/%LOCALE%/customize
browser.customizemode.tip0.shownundefinedfalse
browser.safebrowsing.gethashURLhttp://safebrowsing.clients.google.com/safebrowsing/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2https://safebrowsing.google.com/safebrowsing/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2
browser.safebrowsing.keyURLhttps://sb-ssl.google.com/safebrowsing/newkey?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2undefined
browser.safebrowsing.malware.reportURLhttp://safebrowsing.clients.google.com/safebrowsing/diagnostic?client=%NAME%&hl=%LOCALE%&site=https://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&hl=%LOCALE%&site=
browser.safebrowsing.reportURLhttp://safebrowsing.clients.google.com/safebrowsing/report?https://safebrowsing.google.com/safebrowsing/report?
browser.safebrowsing.updateURLhttp://safebrowsing.clients.google.com/safebrowsing/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2&key=%GOOGLE_API_KEY%https://safebrowsing.google.com/safebrowsing/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2&key=%GOOGLE_API_KEY%
browser.sessionstore.asynctrueundefined
browser.sessionstore.postdata0undefined
browser.tabs.onToptrueundefined
browser.uiCustomization.debugundefinedfalse
browser.uiCustomization.stateundefined
browser.uitour.enabledfalsetrue
browser.uitour.urlundefinedhttps://www.mozilla.org/%LOCALE%/firefox/%VERSION%/tour/
canvas.customfocusring.enabledundefinedfalse
capability.policy.default.Clipboard.cutcopynoAccessundefined
capability.policy.default.Clipboard.pastenoAccessundefined
capability.policy.default.DOMException.codeallAccessundefined
capability.policy.default.DOMException.messageallAccessundefined
capability.policy.default.DOMException.nameallAccessundefined
capability.policy.default.DOMException.resultallAccessundefined
capability.policy.default.DOMException.toString.getallAccessundefined
capability.policy.default.DOMParser.parseFromStreamnoAccessundefined
capability.policy.default.History.back.getallAccessundefined
capability.policy.default.History.currentUniversalXPConnectundefined
capability.policy.default.History.forward.getallAccessundefined
capability.policy.default.History.go.getallAccessundefined
capability.policy.default.History.itemUniversalXPConnectundefined
capability.policy.default.History.nextUniversalXPConnectundefined
capability.policy.default.History.previousUniversalXPConnectundefined
capability.policy.default.History.toStringUniversalXPConnectundefined
capability.policy.default.Location.hash.setallAccessundefined
capability.policy.default.Location.href.setallAccessundefined
capability.policy.default.Location.replace.getallAccessundefined
capability.policy.default.Selection.addSelectionListenerUniversalXPConnectundefined
capability.policy.default.Selection.removeSelectionListenerUniversalXPConnectundefined
capability.policy.default.Window.blur.getallAccessundefined
capability.policy.default.Window.close.getallAccessundefined
capability.policy.default.Window.closed.getallAccessundefined
capability.policy.default.Window.focus.getallAccessundefined
capability.policy.default.Window.frames.getallAccessundefined
capability.policy.default.Window.history.getallAccessundefined
capability.policy.default.Window.length.getallAccessundefined
capability.policy.default.Window.locationallAccessundefined
capability.policy.default.Window.opener.getallAccessundefined
capability.policy.default.Window.parent.getallAccessundefined
capability.policy.default.Window.postMessage.getallAccessundefined
capability.policy.default.Window.self.getallAccessundefined
capability.policy.default.Window.top.getallAccessundefined
capability.policy.default.Window.window.getallAccessundefined
capability.policy.default.XMLHttpRequest.channelnoAccessundefined
capability.policy.default.XMLHttpRequest.getInterfacenoAccessundefined
capability.policy.default.XMLHttpRequest.open-uriallAccessundefined
capability.policy.default_policynamesmailnewsundefined
capability.policy.mailnews.*.attributes.getnoAccessundefined
capability.policy.mailnews.*.baseURI.getnoAccessundefined
capability.policy.mailnews.*.data.getnoAccessundefined
capability.policy.mailnews.*.documentURInoAccessundefined
capability.policy.mailnews.*.getAttributenoAccessundefined
capability.policy.mailnews.*.getAttributeNSnoAccessundefined
capability.policy.mailnews.*.getAttributeNodenoAccessundefined
capability.policy.mailnews.*.getAttributeNodeNSnoAccessundefined
capability.policy.mailnews.*.getNamedItemnoAccessundefined
capability.policy.mailnews.*.getNamedItemNSnoAccessundefined
capability.policy.mailnews.*.host.getnoAccessundefined
capability.policy.mailnews.*.hostname.getnoAccessundefined
capability.policy.mailnews.*.href.getnoAccessundefined
capability.policy.mailnews.*.innerHTML.getnoAccessundefined
capability.policy.mailnews.*.lowSrc.getnoAccessundefined
capability.policy.mailnews.*.nodeValue.getnoAccessundefined
capability.policy.mailnews.*.pathname.getnoAccessundefined
capability.policy.mailnews.*.protocol.getnoAccessundefined
capability.policy.mailnews.*.src.getnoAccessundefined
capability.policy.mailnews.*.substringData.getnoAccessundefined
capability.policy.mailnews.*.text.getnoAccessundefined
capability.policy.mailnews.*.textContentnoAccessundefined
capability.policy.mailnews.*.title.getnoAccessundefined
capability.policy.mailnews.*.wholeTextnoAccessundefined
capability.policy.mailnews.DOMException.toStringnoAccessundefined
capability.policy.mailnews.DOMParser.parseFromStreamnoAccessundefined
capability.policy.mailnews.DOMParser.parseFromStringnoAccessundefined
capability.policy.mailnews.HTMLAnchorElement.toStringnoAccessundefined
capability.policy.mailnews.HTMLDivElement.getAttributesameOriginundefined
capability.policy.mailnews.HTMLDocument.URLnoAccessundefined
capability.policy.mailnews.HTMLDocument.domainnoAccessundefined
capability.policy.mailnews.Location.toStringnoAccessundefined
capability.policy.mailnews.Range.toStringnoAccessundefined
capability.policy.mailnews.SOAPCall.asyncInvokenoAccessundefined
capability.policy.mailnews.SOAPCall.invokenoAccessundefined
capability.policy.mailnews.SOAPCall.transportURInoAccessundefined
capability.policy.mailnews.SOAPCall.verifySourceHeadernoAccessundefined
capability.policy.mailnews.SOAPEncoding.decodenoAccessundefined
capability.policy.mailnews.SOAPEncoding.defaultDecodernoAccessundefined
capability.policy.mailnews.SOAPEncoding.defaultEncodernoAccessundefined
capability.policy.mailnews.SOAPEncoding.encodenoAccessundefined
capability.policy.mailnews.SOAPEncoding.getAssociatedEncodingnoAccessundefined
capability.policy.mailnews.SOAPEncoding.getDecodernoAccessundefined
capability.policy.mailnews.SOAPEncoding.getEncodernoAccessundefined
capability.policy.mailnews.SOAPEncoding.getExternalSchemaURInoAccessundefined
capability.policy.mailnews.SOAPEncoding.getInternalSchemaURInoAccessundefined
capability.policy.mailnews.SOAPEncoding.mapSchemaURInoAccessundefined
capability.policy.mailnews.SOAPEncoding.schemaCollectionnoAccessundefined
capability.policy.mailnews.SOAPEncoding.setDecodernoAccessundefined
capability.policy.mailnews.SOAPEncoding.setEncodernoAccessundefined
capability.policy.mailnews.SOAPEncoding.styleURInoAccessundefined
capability.policy.mailnews.SOAPEncoding.unmapSchemaURInoAccessundefined
capability.policy.mailnews.SOAPFault.detailnoAccessundefined
capability.policy.mailnews.SOAPFault.elementnoAccessundefined
capability.policy.mailnews.SOAPFault.faultActornoAccessundefined
capability.policy.mailnews.SOAPFault.faultCodenoAccessundefined
capability.policy.mailnews.SOAPFault.faultNamespaceURInoAccessundefined
capability.policy.mailnews.SOAPFault.faultStringnoAccessundefined
capability.policy.mailnews.SOAPHeaderBlock.actorURInoAccessundefined
capability.policy.mailnews.SOAPHeaderBlock.mustUnderstandnoAccessundefined
capability.policy.mailnews.SOAPParameternoAccessundefined
capability.policy.mailnews.SOAPPropertyBagMutator.addPropertynoAccessundefined
capability.policy.mailnews.SOAPPropertyBagMutator.propertyBagnoAccessundefined
capability.policy.mailnews.SOAPResponse.faultnoAccessundefined
capability.policy.mailnews.SchemaLoader.loadnoAccessundefined
capability.policy.mailnews.SchemaLoader.loadAsyncnoAccessundefined
capability.policy.mailnews.SchemaLoader.onErrornoAccessundefined
capability.policy.mailnews.SchemaLoader.onLoadnoAccessundefined
capability.policy.mailnews.SchemaLoader.processSchemaElementnoAccessundefined
capability.policy.mailnews.WSDLLoader.loadnoAccessundefined
capability.policy.mailnews.WSDLLoader.loadAsyncnoAccessundefined
capability.policy.mailnews.WSDLLoader.onErrornoAccessundefined
capability.policy.mailnews.WSDLLoader.onLoadnoAccessundefined
capability.policy.mailnews.WebServiceProxyFactory.createProxynoAccessundefined
capability.policy.mailnews.WebServiceProxyFactory.createProxyAsyncnoAccessundefined
capability.policy.mailnews.WebServiceProxyFactory.onErrornoAccessundefined
capability.policy.mailnews.WebServiceProxyFactory.onLoadnoAccessundefined
capability.policy.mailnews.Window.blurnoAccessundefined
capability.policy.mailnews.Window.focusnoAccessundefined
capability.policy.mailnews.Window.innerHeight.setnoAccessundefined
capability.policy.mailnews.Window.innerWidth.setnoAccessundefined
capability.policy.mailnews.Window.moveBynoAccessundefined
capability.policy.mailnews.Window.moveTonoAccessundefined
capability.policy.mailnews.Window.name.setnoAccessundefined
capability.policy.mailnews.Window.outerHeight.setnoAccessundefined
capability.policy.mailnews.Window.outerWidth.setnoAccessundefined
capability.policy.mailnews.Window.resizeBynoAccessundefined
capability.policy.mailnews.Window.resizeTonoAccessundefined
capability.policy.mailnews.Window.screenX.setnoAccessundefined
capability.policy.mailnews.Window.screenY.setnoAccessundefined
capability.policy.mailnews.Window.sizeToContentnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.abortnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.channelnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.getAllResponseHeadersnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.getInterfacenoAccessundefined
capability.policy.mailnews.XMLHttpRequest.getResponseHeadernoAccessundefined
capability.policy.mailnews.XMLHttpRequest.onerrornoAccessundefined
capability.policy.mailnews.XMLHttpRequest.onloadnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.onreadystatechangenoAccessundefined
capability.policy.mailnews.XMLHttpRequest.opennoAccessundefined
capability.policy.mailnews.XMLHttpRequest.overrideMimeTypenoAccessundefined
capability.policy.mailnews.XMLHttpRequest.readyStatenoAccessundefined
capability.policy.mailnews.XMLHttpRequest.responseTextnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.responseXMLnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.sendnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.setRequestHeadernoAccessundefined
capability.policy.mailnews.XMLHttpRequest.statusnoAccessundefined
capability.policy.mailnews.XMLHttpRequest.statusTextnoAccessundefined
capability.policy.mailnews.XMLSerializer.serializeToStreamnoAccessundefined
capability.policy.mailnews.XMLSerializer.serializeToStringnoAccessundefined
capability.policy.mailnews.document.loadnoAccessundefined
capability.policy.mailnews.sitesmailbox: imap: news:undefined
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.debugger.auto-pretty-printundefinedfalse
devtools.debugger.tracerundefinedfalse
devtools.editor.autoclosebracketsundefinedtrue
devtools.editor.keymapundefineddefault
devtools.netmonitor.statisticsundefinedtrue
devtools.scratchpad.enableCodeFoldingundefinedtrue
devtools.scratchpad.showTrailingSpaceundefinedfalse
devtools.styleeditor.autocompletion-enabledundefinedtrue
devtools.styleeditor.source-maps-enabledundefinedfalse
devtools.styleeditor.transitionstrueundefined
dom.cycle_collector.incrementalundefinedfalse
dom.disable_beforeunloadundefinedfalse
dom.forms.colorfalsetrue
dom.forms.numberfalsetrue
dom.gamepad.enabledfalsetrue
dom.promise.enabledfalseundefined
dom.workers.sharedWorkers.enabledfalsetrue
extensions.checkCompatibility.temporaryThemeOverride_minAppVersionundefined29.0a1
font.default.x-user-defserifundefined
font.minimum-size.x-user-def0undefined
font.name.monospace.x-user-defmonospaceundefined
font.name.sans-serif.x-user-defsans-serifundefined
font.name.serif.x-user-defserifundefined
font.size.fixed.x-user-def12undefined
font.size.variable.x-user-def16undefined
identity.fxaccounts.auth.urihttps://api-accounts.dev.lcip.org/v1https://api.accounts.firefox.com/v1
identity.fxaccounts.remote.force_auth.uriundefinedhttps://accounts.firefox.com/force_auth?service=sync&context=fx_desktop_v1
identity.fxaccounts.remote.signin.uriundefinedhttps://accounts.firefox.com/signin?service=sync&context=fx_desktop_v1
identity.fxaccounts.remote.signup.uriundefinedhttps://accounts.firefox.com/signup?service=sync&context=fx_desktop_v1
identity.fxaccounts.settings.uriundefinedhttps://accounts.firefox.com/settings
intl.ime.use_simple_context_on_password_fieldundefinedtrue
intl.uidirection.ugundefinedrtl
javascript.options.jit_hardeningtrueundefined
layers.async-video.enabledfalsetrue
layers.offmainthreadcomposition.frame-rateundefined-1
layers.prefer-memory-over-shmemtrueundefined
layers.use-deprecated-texturestruefalse
layout.css.overflow-clip-box.enabledundefinedfalse
layout.css.touch_action.enabledundefinedfalse
layout.css.variables.enabledundefinedfalse
layout.css.will-change.enabledundefinedfalse
media.encoder.webm.enabledundefinedtrue
media.peerconnection.capture_delayundefined70
media.webvtt.enabledfalsetrue
media.webvtt.regions.enabledundefinedfalse
memory.blob_report.stack_framesundefined0
memory.system_memory_reporterundefinedfalse
network.dnsCacheEntriesundefined400
network.dnsCacheExpirationundefined60
network.dnsCacheExpirationGracePeriod259200060
network.http.response.timeoutundefined300
network.http.spdy.enabled.http2draftundefinedfalse
network.http.spdy.enforce-tls-profileundefinedtrue
network.http.spdy.push-allowance65536131072
network.seer.enabledfalsetrue
network.seer.max-db-sizeundefined157286400
network.seer.preserveundefined80
plugins.enumerable_names*Java,Nexus Personal,QuickTime,Shockwave
security.ssl.enable_alpnundefinedfalse
security.ssl.enable_npnundefinedtrue
services.sync.fxa.privacyURLundefinedhttps://accounts.firefox.com/legal/privacy
services.sync.fxa.termsURLundefinedhttps://accounts.firefox.com/legal/terms
services.sync.log.logger.declinedundefinedDebug
services.sync.log.logger.identityundefinedDebug
services.sync.scheduler.fxa.singleDeviceIntervalundefined3600
services.sync.scheduler.singleDeviceInterval86400undefined
services.sync.scheduler.sync11.singleDeviceIntervalundefined86400
services.sync.tokenServerURIundefinedhttps://token.services.mozilla.com/1.0/sync/1.5
signon.overrideAutocompleteundefinedfalse
signon.useDOMFormHasPasswordtrueundefined
social.allowMultipleWorkerstrueundefined
ui.key.menuAccessKeyFocusesfalsetrue
urlclassifier.downloadAllowTableundefined
urlclassifier.downloadBlockTableundefinedgoog-badbinurl-shavar
urlclassifier.download_allow_tablegoog-downloadwhite-digest256undefined
urlclassifier.download_block_tablegoog-badbinurl-shavarundefined