This is difference between the default preference of Firefox 39.0 beta 3 and one of 38.0.5.

Default pref Javascript files (Firefox 39.0 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 40.0 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 39.0Firefox 40.0 beta 3
app.update.badgeWaitTimeundefined345600
apz.fling_repaint_interval7516
apz.pan_repaint_interval25016
apz.smooth_scroll_repaint_interval7516
apz.x_skate_size_multiplier1.52.5
apz.y_skate_size_multiplier2.53.5
browser.devedition.theme.enabledfalseundefined
browser.devedition.theme.showCustomizeButtonfalseundefined
browser.newtabpage.updateIntroShownundefinedfalse
browser.push.warning.infoURLundefinedhttps://www.mozilla.org/%LOCALE%/firefox/push/
browser.search.jarURIsundefinedchrome://browser/locale/searchplugins/
browser.search.loadFromJarsundefinedtrue
browser.sessionstore.upgradeBackup.maxUpgradeBackupsundefined3
browser.uitour.readerViewTrigger^https://www.mozilla.org/[^/]+/firefox/reading/start^https:\/\/www\.mozilla\.org\/[^\/]+\/firefox\/reading\/start
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.browserconsole.filter.serviceworkersundefinedtrue
devtools.browserconsole.filter.sharedworkersundefinedtrue
devtools.browserconsole.filter.windowlessworkersundefinedtrue
devtools.command-button-frames.enabledfalsetrue
devtools.command-button-rulers.enabledundefinedfalse
devtools.debugger.chrome-enabledtrueundefined
devtools.devedition.promo.enabledfalsetrue
devtools.gcli.imgurClientIDundefined0df414e888d7240
devtools.gcli.imgurUploadURLundefinedhttps://api.imgur.com/3/image
devtools.inspector.mdnDocsTooltip.enabledundefinedtrue
devtools.performance.enabledundefinedtrue
devtools.performance.memory.max-log-length2147483647125000
devtools.performance.profiler.buffer-sizeundefined10000000
devtools.performance.profiler.sample-frequency-khzundefined1
devtools.performance.ui.experimentalundefinedfalse
devtools.performance.ui.show-jit-optimizationsundefinedfalse
devtools.performance_dev.enabledfalseundefined
devtools.profiler.enabledtrueundefined
devtools.profiler.ui.flatten-tree-recursiontrueundefined
devtools.profiler.ui.show-idle-blockstrueundefined
devtools.profiler.ui.show-platform-datafalseundefined
devtools.scratchpad.editorFontSizeundefined12
devtools.scratchpad.lineNumbersundefinedtrue
devtools.scratchpad.wrapTextundefinedfalse
devtools.timeline.enabledfalseundefined
devtools.timeline.hiddenMarkers[]undefined
devtools.toolbox.toolbarSpec["splitconsole", "paintflashing toggle","tilt toggle","scratchpad","resize toggle","eyedropper","screenshot --fullpage"]["splitconsole", "paintflashing toggle","tilt toggle","scratchpad","resize toggle","eyedropper","screenshot --fullpage", "rulers"]
devtools.webconsole.filter.serviceworkersundefinedfalse
devtools.webconsole.filter.sharedworkersundefinedfalse
devtools.webconsole.filter.windowlessworkersundefinedfalse
dom.apps.reset-permissionsundefinedfalse
dom.compositionevent.allow_control_charactersundefinedfalse
dom.ipc.plugins.asyncInitfalsetrue
dom.ipc.plugins.contentTimeoutSecs4510
dom.ipc.shims.enabledWarningsundefinedfalse
dom.presentation.tcp_server.debugundefinedfalse
dom.push.adaptive.enabledundefinedfalse
dom.push.adaptive.gapundefined60000
dom.push.adaptive.lastGoodPingIntervalundefined180000
dom.push.adaptive.lastGoodPingInterval.mobileundefined180000
dom.push.adaptive.lastGoodPingInterval.wifiundefined180000
dom.push.adaptive.upperLimitundefined1740000
dom.push.connection.enabledundefinedtrue
dom.push.debugundefinedfalse
dom.push.enabledundefinedfalse
dom.push.pingIntervalundefined1800000
dom.push.pingInterval.defaultundefined180000
dom.push.pingInterval.mobileundefined180000
dom.push.pingInterval.wifiundefined180000
dom.push.requestTimeoutundefined10000
dom.push.retryBaseIntervalundefined5000
dom.push.serverURLundefinedwss://push.services.mozilla.com/
dom.push.udp.wakeupEnabledundefinedfalse
dom.push.userAgentIDundefined
dom.webcrypto.enabledtrueundefined
extensions.blocklist.suppressUIundefinedtrue
extensions.getAddons.link.urlundefinedhttps://addons.mozilla.org/%LOCALE%/firefox/
extensions.hotfix.certs.2.sha1Fingerprintundefined39:E7:2B:7A:5B:CF:37:78:F9:5D:4A:E0:53:2D:2F:3D:68:53:C5:60
gfx.vsync.compositorundefinedtrue
gfx.vsync.hw-vsync.enabledundefinedtrue
gfx.vsync.refreshdriverundefinedtrue
gl.require-hardwareundefinedfalse
identity.fxaccounts.remote.webchannel.uriundefinedhttps://accounts.firefox.com/
image.downscale-during-decode.enabledfalsetrue
layers.offmainthreadcomposition.enabledfalsetrue
lightweightThemes.recommendedThemes[{"id":"recommended-1","homepageURL":"https://addons.mozilla.org/firefox/addon/a-web-browser-renaissance/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/1.header.jpg","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/1.footer.jpg","textcolor":"#000000","accentcolor":"#f2d9b1","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/1.icon.jpg","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/1.preview.jpg","author":"Sean.Martell","version":"0"},{"id":"recommended-2","homepageURL":"https://addons.mozilla.org/firefox/addon/space-fantasy/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/2.header.jpg","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/2.footer.jpg","textcolor":"#ffffff","accentcolor":"#d9d9d9","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/2.icon.jpg","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/2.preview.jpg","author":"fx5800p","version":"1.0"},{"id":"recommended-3","homepageURL":"https://addons.mozilla.org/firefox/addon/linen-light/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/3.header.png","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/3.footer.png","textcolor":"#None","accentcolor":"#ada8a8","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/3.icon.png","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/3.preview.png","author":"DVemer","version":"1.0"},{"id":"recommended-4","homepageURL":"https://addons.mozilla.org/firefox/addon/pastel-gradient/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/4.header.png","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/4.footer.png","textcolor":"#000000","accentcolor":"#000000","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/4.icon.png","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/4.preview.png","author":"darrinhenein","version":"1.0"},{"id":"recommended-5","homepageURL":"https://addons.mozilla.org/firefox/addon/carbon-light/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/5.header.png","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/5.footer.png","textcolor":"#3b3b3b","accentcolor":"#2e2e2e","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/5.icon.jpg","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/5.preview.jpg","author":"Jaxivo","version":"1.0"}][{"id":"recommended-1","homepageURL":"https://addons.mozilla.org/firefox/addon/a-web-browser-renaissance/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/1.header.jpg","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/1.footer.jpg","textcolor":"#000000","accentcolor":"#f2d9b1","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/1.icon.jpg","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/1.preview.jpg","author":"Sean.Martell","version":"0"},{"id":"recommended-2","homepageURL":"https://addons.mozilla.org/firefox/addon/space-fantasy/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/2.header.jpg","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/2.footer.jpg","textcolor":"#ffffff","accentcolor":"#d9d9d9","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/2.icon.jpg","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/2.preview.jpg","author":"fx5800p","version":"1.0"},{"id":"recommended-3","homepageURL":"https://addons.mozilla.org/firefox/addon/linen-light/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/3.header.png","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/3.footer.png","accentcolor":"#ada8a8","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/3.icon.png","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/3.preview.png","author":"DVemer","version":"1.0"},{"id":"recommended-4","homepageURL":"https://addons.mozilla.org/firefox/addon/pastel-gradient/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/4.header.png","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/4.footer.png","textcolor":"#000000","accentcolor":"#000000","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/4.icon.png","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/4.preview.png","author":"darrinhenein","version":"1.0"},{"id":"recommended-5","homepageURL":"https://addons.mozilla.org/firefox/addon/carbon-light/","headerURL":"resource:///chrome/browser/content/browser/defaultthemes/5.header.png","footerURL":"resource:///chrome/browser/content/browser/defaultthemes/5.footer.png","textcolor":"#3b3b3b","accentcolor":"#2e2e2e","iconURL":"resource:///chrome/browser/content/browser/defaultthemes/5.icon.jpg","previewURL":"resource:///chrome/browser/content/browser/defaultthemes/5.preview.jpg","author":"Jaxivo","version":"1.0"}]
lightweightThemes.selectedThemeIDundefined
loop.contextInConverations.enabledfalseundefined
loop.contextInConversations.enabledundefinedtrue
mathml.scale_stretchy_operators.enabledundefinedtrue
media.decoder.heuristic.dormant.enabledundefinedtrue
media.decoder.heuristic.dormant.timeoutundefined60000
media.getusermedia.screensharing.allowed_domainswebex.com,*.webex.com,ciscospark.com,*.ciscospark.com,projectsquared.com,*.projectsquared.com,*.room.co,room.co,beta.talky.io,talky.io,*.clearslide.com,appear.in,*.appear.in,tokbox.com,*.tokbox.com,*.sso.francetelecom.fr,*.si.francetelecom.fr,*.sso.infra.ftgroup,*.multimedia-conference.orange-business.com,*.espacecollaboration.orange-business.com,free.gotomeeting.com,g2m.me,*.g2m.me,example.com,*.mypurecloud.com,*.mypurecloud.com.auwebex.com,*.webex.com,ciscospark.com,*.ciscospark.com,projectsquared.com,*.projectsquared.com,*.room.co,room.co,beta.talky.io,talky.io,*.clearslide.com,appear.in,*.appear.in,tokbox.com,*.tokbox.com,*.sso.francetelecom.fr,*.si.francetelecom.fr,*.sso.infra.ftgroup,*.multimedia-conference.orange-business.com,*.espacecollaboration.orange-business.com,free.gotomeeting.com,g2m.me,*.g2m.me,example.com,*.mypurecloud.com,*.mypurecloud.com.au,spreed.me,*.spreed.me,*.spreed.com
media.hardware-video-decoding.enabledundefinedtrue
media.peerconnection.identity.enabledfalsetrue
memory.report_concurrencyundefined10
network.auth.allow-subresource-authundefined1
network.http.atsvc.enabledtrueundefined
network.http.atsvc.oetrueundefined
network.offline-mirrors-connectivityundefinedtrue
plugins.testmodeundefinedfalse
security.apps.privileged.CSP.defaultdefault-src *; script-src 'self'; object-src 'none'; style-src 'self' 'unsafe-inline'default-src * data: blob:; script-src 'self'; object-src 'none'; style-src 'self' 'unsafe-inline'
security.onecrl.maximum_staleness_in_secondsundefined0
selectioncaret.detects.longtapundefinedtrue
services.push.enabledfalseundefined
services.push.serverURLwss://push.services.mozilla.com/undefined
social.share.activationPanelEnabledfalsetrue
toolbar.customization.usesheetfalseundefined
toolkit.telemetry.archive.enabledundefinedtrue
toolkit.telemetry.unifiedundefinedtrue
urlclassifier.disallow_completionstest-malware-simple,test-phish-simple,goog-downloadwhite-digest256,mozpub-track-digest256test-malware-simple,test-phish-simple,test-unwanted-simple,goog-downloadwhite-digest256,mozpub-track-digest256
urlclassifier.malwareTablegoog-malware-shavar,test-malware-simplegoog-malware-shavar,goog-unwanted-shavar,test-malware-simple,test-unwanted-simple
xpinstall.customConfirmationUIfalsetrue
xpinstall.signatures.devInfoURLundefinedhttps://wiki.mozilla.org/Addons/Extension_Signing
xpinstall.signatures.requiredundefinedfalse