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

Default pref Javascript files (Firefox 38.0.5 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 39.0 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 38.0.5Firefox 39.0 beta 3
browser.addon-watch.ignoreundefined["mochikit@mozilla.org","special-powers@mozilla.org","fxdevtools-adapters@mozilla.org","fx-devtools"]
browser.addon-watch.intervalundefined-1
browser.addon-watch.percentage-limitundefined5
browser.eme.ui.enabledfalsetrue
browser.newtabpage.directory.pinghttps://tiles.services.mozilla.com/v2/links/https://tiles.services.mozilla.com/v3/links/
browser.newtabpage.directory.sourcehttps://tiles.services.mozilla.com/v2/links/fetch/%LOCALE%https://tiles.services.mozilla.com/v3/links/fetch/%LOCALE%/%CHANNEL%
browser.safebrowsing.downloads.remote.enabledfalsetrue
converter.html2txt.always_include_rubyundefinedfalse
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.devedition.promo.enabledfalsetrue
devtools.devices.urlundefinedhttps://code.cdn.mozilla.net/devices/devices.json
devtools.performance.memory.max-log-lengthundefined2147483647
devtools.performance.memory.sample-probabilityundefined0.05
devtools.performance.timeline.hidden-markersundefined[]
devtools.remote.wifi.scanfalsetrue
devtools.remote.wifi.visiblefalsetrue
devtools.serviceWorkers.testing.enabledundefinedfalse
devtools.webconsole.inputHistoryCountundefined50
dom.caches.enabledundefinedfalse
dom.fetch.enabledfalseundefined
dom.ipc.plugins.asyncInitfalsetrue
gfx.downloadable_fonts.disable_cacheundefinedfalse
gfx.downloadable_fonts.woff2.enabledfalsetrue
image.decode-immediately.enabledundefinedfalse
image.decode-only-on-draw.enabledundefinedtrue
image.mem.decodeondrawtrueundefined
javascript.options.compact_on_user_inactiveundefinedtrue
layout.async-containerless-scrolling.enabledtrueundefined
layout.css.expensive-style-struct-assertions.enabledundefinedfalse
layout.css.scroll-snap.enabledundefinedtrue
layout.css.scroll-snap.prediction-max-velocityundefined2000
layout.css.scroll-snap.prediction-sensitivityundefined0.750
layout.css.scroll-snap.proximity-thresholdundefined200
layout.css.unprefixing-service.enabledundefinedtrue
layout.float-fragments-inside-column.enabledfalsetrue
loop.CSPdefault-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:default-src 'self' about: file: chrome:; img-src * data:; 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.contextInConverations.enabledundefinedfalse
loop.debug.twoWayMediaTelemetryundefinedfalse
loop.screenshare.enabledtrueundefined
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.comwebex.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
network.dns.offline-localhostundefinedtrue
network.http.altsvc.enabledfalsetrue
network.http.altsvc.oefalsetrue
network.http.atsvc.enabledfalsetrue
network.http.atsvc.oefalsetrue
network.http.enforce-framing.softundefinedtrue
network.standard-url.max-lengthundefined1048576
network.sts.max_time_for_events_between_two_pollsundefined100
network.sts.serve_multiple_events_per_poll_iterationundefinedtrue
print.enable_e10s_testingfalseundefined
privacy.trackingprotection.pbmode.enabledundefinedfalse
services.sync.prefs.sync.lightweightThemes.isThemeSelectedtrueundefined
services.sync.prefs.sync.lightweightThemes.selectedThemeIDundefinedtrue
xpinstall.customConfirmationUIundefinedfalse