This is difference between the default preference of Firefox 43.0 beta 3 and one of 42.0.

Default pref Javascript files (Firefox 42.0 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 43.0 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 42.0Firefox 43.0 beta 3
apz.highlight_checkerboarded_areasundefinedfalse
browser.safebrowsing.appRepURLhttps://sb-ssl.google.com/safebrowsing/clientreport/download?key=%GOOGLE_API_KEY%undefined
browser.safebrowsing.gethashURLhttps://safebrowsing.google.com/safebrowsing/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2undefined
browser.safebrowsing.malware.reportURLhttps://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&hl=%LOCALE%&site=undefined
browser.safebrowsing.provider.google.appRepURLundefinedhttps://sb-ssl.google.com/safebrowsing/clientreport/download?key=%GOOGLE_API_KEY%
browser.safebrowsing.provider.google.gethashURLundefinedhttps://safebrowsing.google.com/safebrowsing/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2
browser.safebrowsing.provider.google.listsundefinedgoog-badbinurl-shavar,goog-downloadwhite-digest256,goog-phish-shavar,goog-malware-shavar,goog-unwanted-shavar
browser.safebrowsing.provider.google.reportURLundefinedhttps://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&hl=%LOCALE%&site=
browser.safebrowsing.provider.google.updateURLundefinedhttps://safebrowsing.google.com/safebrowsing/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2&key=%GOOGLE_API_KEY%
browser.safebrowsing.provider.mozilla.gethashURLundefinedhttps://shavar.services.mozilla.com/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2
browser.safebrowsing.provider.mozilla.listsundefinedmozstd-track-digest256,mozstd-trackwhite-digest256,mozfull-track-digest256
browser.safebrowsing.provider.mozilla.lists.mozfull.descriptionundefinedmozfullDesc
browser.safebrowsing.provider.mozilla.lists.mozfull.nameundefinedmozfullName
browser.safebrowsing.provider.mozilla.lists.mozstd.descriptionundefinedmozstdDesc
browser.safebrowsing.provider.mozilla.lists.mozstd.nameundefinedmozstdName
browser.safebrowsing.provider.mozilla.updateURLundefinedhttps://shavar.services.mozilla.com/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2
browser.safebrowsing.updateURLhttps://safebrowsing.google.com/safebrowsing/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2&key=%GOOGLE_API_KEY%undefined
browser.search.jarURIschrome://browser/locale/searchplugins/undefined
browser.search.loadFromJarstrueundefined
browser.search.showOneOffButtonstrueundefined
browser.shell.defaultBrowserCheckCountundefined0
browser.shell.skipDefaultBrowserCheckundefinedtrue
browser.trackingprotection.gethashURLhttps://shavar.services.mozilla.com/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2undefined
browser.trackingprotection.updateURLhttps://shavar.services.mozilla.com/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2undefined
browser.urlbar.unifiedcompletefalsetrue
browser.zoom.reflowOnZoomfalseundefined
browser.zoom.reflowZoom.reflowTextOnPageLoadtrueundefined
browser.zoom.reflowZoom.reflowTimeout500undefined
canvas.capturestream.enabledfalsetrue
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.browserconsole.filter.servererrorundefinedfalse
devtools.browserconsole.filter.serverinfoundefinedfalse
devtools.browserconsole.filter.serverlogundefinedfalse
devtools.browserconsole.filter.serverwarnundefinedfalse
devtools.devedition.promo.enabledfalsetrue
devtools.hud.loglimit.console2001000
devtools.hud.loglimit.cssparser2001000
devtools.hud.loglimit.exception2001000
devtools.hud.loglimit.network2001000
devtools.inspector.animationInspectorV3falseundefined
devtools.memory.enabledundefinedfalse
devtools.performance.timeline.hidden-markers[]["Composite","CompositeForwardTransaction"]
devtools.performance.ui.enable-memory-flameundefinedfalse
devtools.performance.ui.show-triggers-for-gc-typesundefinedTOO_MUCH_MALLOC ALLOC_TRIGGER LAST_DITCH EAGER_ALLOC_TRIGGER
devtools.telemetry.supported_performance_marksundefinedcontentInteractive,navigationInteractive,navigationLoaded,visuallyLoaded,fullyLoaded,mediaEnumerated,scanEnd
devtools.webconsole.filter.servererrorundefinedfalse
devtools.webconsole.filter.serverinfoundefinedfalse
devtools.webconsole.filter.serverlogundefinedfalse
devtools.webconsole.filter.serverwarnundefinedfalse
dom.fileHandle.enabledundefinedtrue
dom.messageChannel.enabledtrueundefined
dom.mozKillSwitch.enabledundefinedfalse
dom.presentation.discovery.timeout_msundefined10000
dom.select_events.enabledundefinedfalse
font.default.x-mathundefinedserif
font.size.fixed.x-mathundefined13
font.size.variable.x-mathundefined16
full-screen-api.warning.delayundefined500
full-screen-api.warning.timeoutundefined3000
gfx.vsync.compositortrueundefined
gfx.vsync.hw-vsync.enabledtrueundefined
gfx.vsync.refreshdrivertrueundefined
image.high_quality_downscaling.enabledtrueundefined
image.high_quality_downscaling.min_factor1000undefined
image.high_quality_upscaling.max_size20971520undefined
intl.ime.use_simple_context_on_password_fieldtruefalse
layout.css.control-characters.visibleundefinedfalse
layout.css.will-change.enabledtrueundefined
layout.spammy_warnings.enabledtruefalse
loop.linkClicker.urlundefinedhttps://hello.firefox.com/
media.audio_data.enabledfalseundefined
media.fragmented-mp4.exposedfalseundefined
media.fragmented-mp4.ffmpeg.enabledfalsetrue
media.gmp.trial-create.enabledundefinedtrue
media.mediasource.enabledfalsetrue
media.mediasource.format-readertrueundefined
media.mediasource.format-reader.webmfalseundefined
media.mediasource.webm.enabledfalsetrue
media.peerconnection.ice.tcp_so_sock_countundefined0
media.useAudioChannelServicetrueundefined
network.IDN.use_whitelisttruefalse
network.http.packaged-apps-developer-modeundefinedfalse
network.http.use-cachetrueundefined
network.sts.serve_multiple_events_per_poll_iterationtrueundefined
pfs.datasource.urlhttps://pfs.mozilla.org/plugins/PluginFinderService.php?mimetype=%PLUGIN_MIMETYPE%&appID=%APP_ID%&appVersion=%APP_VERSION%&clientOS=%CLIENT_OS%&chromeLocale=%CHROME_LOCALE%&appRelease=%APP_RELEASE%undefined
security.onecrl.maximum_staleness_in_seconds0108000
security.pki.sha1_enforcement_levelundefined2
security.sri.enableundefinedtrue
security.ssl.warn_missing_rfc57461undefined
services.sync.prefs.sync.privacy.trackingprotection.pbmode.enabledundefinedtrue
signon.rememberSignons.visibilityTogglefalsetrue
svg.transform-box.enabledundefinedfalse
svg.transform-origin.enabledfalseundefined
toolkit.telemetry.optoutSampleundefinedtrue
urlclassifier.disallow_completionstest-malware-simple,test-phish-simple,test-unwanted-simple,test-track-simple,test-trackwhite-simple,goog-downloadwhite-digest256,mozstd-track-digest256,mozstd-trackwhite-digest256test-malware-simple,test-phish-simple,test-unwanted-simple,test-track-simple,test-trackwhite-simple,goog-downloadwhite-digest256,mozstd-track-digest256,mozstd-trackwhite-digest256,mozfull-track-digest256
xpinstall.signatures.requiredfalsetrue