This is difference between the default preference of Firefox 38.0 beta 1 and one of 37.0.1.

Default pref Javascript files (Firefox 37.0.1 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 38.0 beta 1 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 37.0.1Firefox 38.0 beta 1
accessibility.ipc_architecture.enabledundefinedtrue
apz.subframe.enabledfalseundefined
browser.flash-protected-mode-flip.doneundefinedfalse
browser.flash-protected-mode-flip.enableundefinedfalse
browser.preferences.inContentfalsetrue
browser.readinglist.enabledundefinedtrue
browser.readinglist.sidebarEverOpenedundefinedfalse
browser.search.highlightCount0undefined
browser.search.param.yahoo-frmoz35undefined
browser.search.param.yahoo-fr-jamozffundefined
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.browserconsole.filter.netxhrundefinedfalse
devtools.debugger.log.verboseundefinedfalse
devtools.devedition.promo.enabledfalsetrue
devtools.inspector.showAllAnonymousContentundefinedfalse
devtools.performance.ui.enable-framerateundefinedtrue
devtools.performance.ui.enable-memoryundefinedfalse
devtools.performance.ui.flatten-tree-recursionundefinedtrue
devtools.performance.ui.invert-call-treeundefinedtrue
devtools.performance.ui.invert-flame-graphundefinedfalse
devtools.performance.ui.show-idle-blocksundefinedtrue
devtools.performance.ui.show-platform-dataundefinedfalse
devtools.performance.ui.show-timeline-memoryfalseundefined
devtools.profiler.ui.flatten-tree-recursionundefinedtrue
devtools.profiler.ui.show-idle-blocksundefinedtrue
devtools.webaudioeditor.inspectorWidthundefined300
devtools.webconsole.filter.netxhrundefinedfalse
dom.broadcastChannel.enabledundefinedtrue
dom.enable_user_timingundefinedtrue
dom.image.picture.enabledfalsetrue
dom.image.srcset.enabledfalsetrue
dom.indexedDB.warningQuota50undefined
dom.ipc.cpow.timeout0500
dom.ipc.plugins.contentTimeoutSecsundefined45
dom.ipc.processHangMonitorundefinedtrue
dom.ipc.reportProcessHangsundefinedtrue
dom.keyboardevent.code.enabledfalsetrue
dom.keyboardevent.dispatch_during_compositionundefinedfalse
dom.max_child_script_run_timeundefined10
dom.performance.enable_user_timing_loggingundefinedfalse
dom.presentation.enabledundefinedfalse
dom.requestSync.enabledundefinedfalse
dom.secureelement.enabledundefinedfalse
dom.url.encode_decode_hashundefinedfalse
dom.workers.websocket.enabledfalsetrue
identity.fxaccounts.remote.oauth.uriundefinedhttps://oauth.accounts.firefox.com/v1
identity.fxaccounts.remote.profile.uriundefinedhttps://profile.accounts.firefox.com/v1
image.mem.decodeondrawfalsetrue
javascript.options.mem.gc_compactingundefinedtrue
layout.css.ruby.enabledfalsetrue
layout.scroll.root-frame-containersundefinedfalse
loop.CSPdefault-src 'self' about: file: chrome:; img-src 'self' data: http://www.gravatar.com/ about: file: chrome:; font-src 'none'; connect-src wss://*.tokbox.com https://*.opentok.com https://*.tokbox.com wss://*.mozilla.com https://*.mozilla.org wss://*.mozaws.net; media-src blob:default-src 'self' about: file: chrome:; img-src 'self' data: https://www.gravatar.com/ about: file: chrome:; font-src 'none'; connect-src wss://*.tokbox.com https://*.opentok.com https://*.tokbox.com wss://*.mozilla.com https://*.mozilla.org wss://*.mozaws.net; media-src blob:
loop.browserSharing.showInfoBarundefinedtrue
loop.contacts.gravatars.promoundefinedtrue
loop.contacts.gravatars.showundefinedfalse
loop.rooms.enabledtrueundefined
loop.screenshare.enabledundefinedtrue
media.eme.apiVisiblefalsetrue
media.eme.enabledfalsetrue
media.gmp-eme-adobe.hiddentrueundefined
media.gmp.insecure.allowundefinedfalse
media.mediasource.mp4.enabledfalsetrue
media.mediasource.webm.enabledtruefalse
media.mediasource.whitelistundefinedtrue
media.mediasource.youtubeonlytrueundefined
media.peerconnection.default_iceservers[{"url": "stun:stun.services.mozilla.com"}][{"urls": ["stun:stun.services.mozilla.com"]}]
network.http.altsvc.enabledfalsetrue
network.http.altsvc.oefalsetrue
network.http.atsvc.enabledfalseundefined
network.http.atsvc.oefalseundefined
network.http.spdy.default-concurrentundefined100
network.predictor.cleaned-upundefinedfalse
network.predictor.enabledfalsetrue
network.predictor.max-db-size157286400undefined
network.predictor.max-queue-size50undefined
network.predictor.max-resources-per-entryundefined100
network.predictor.preserve80undefined
permissions.manager.defaultsUrlundefinedresource://app/defaults/permissions
reader.color_schemeundefinedlight
reader.color_scheme.valuesundefined["light","dark","sepia"]
reader.font_sizeundefined5
reader.font_typeundefinedsans-serif
reader.has_used_toolbarundefinedfalse
reader.parse-on-load.enabledundefinedtrue
reader.parse-on-load.force-enabledundefinedfalse
reader.toolbar.verticalundefinedtrue
readinglist.scheduler.enabledundefinedtrue
readinglist.serverundefinedhttps://readinglist.services.mozilla.com/v1
security.ssl.allow_unrestricted_renego_everywhere__temporarily_available_preffalseundefined
security.ssl.renego_unrestricted_hostsundefined
security.tls.unrestricted_rc4_fallbackundefinedtrue
services.sync.enabledundefinedtrue
social.share.activationPanelEnabledfalsetrue
svg.svg-iframe.enabledfalseundefined
ui.menu.incremental_search.timeoutundefined1000
webgl.bypass-shader-validationundefinedfalse
webgl.enable-prototype-webgl2undefinedfalse
webgl.shader_validatortrueundefined