This is difference between the default preference of Firefox 31.0 beta 6 and one of 30.0.

Default pref Javascript files (Firefox 30.0 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 31.0 beta 6 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 30.0Firefox 31.0 beta 6
browser.cache.memory_limit51200undefined
browser.slowStartup.timeThreshold6000050000
browser.tabs.closeButtons1undefined
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.browserconsole.filter.cssparsertruefalse
devtools.browserconsole.filter.networkinfotruefalse
devtools.editor.tabsize42
devtools.netmonitor.panes-network-details-width450550
devtools.toolbox.toolbarSpec["splitconsole", "paintflashing toggle","tilt toggle","scratchpad","resize toggle"]["splitconsole", "paintflashing toggle","tilt toggle","scratchpad","resize toggle","eyedropper"]
devtools.webconsole.filter.jslogtruefalse
dom.mozAlarms.enabledfalsetrue
dom.network.enabledtrueundefined
dom.network.meteredfalseundefined
dom.sysmsg.enabledfalsetrue
font.mathfont-familyMathJax_Main, STIXNonUnicode, STIXSizeOneSym, STIXSize1, STIXGeneral, Asana Math, Standard Symbols L, DejaVu Sans, Cambria MathLatin Modern Math, XITS Math, STIX Math, Cambria Math, Asana Math, TeX Gyre Bonum Math, TeX Gyre Pagella Math, TeX Gyre Termes Math, Neo Euler, Lucida Bright Math, MathJax_Main, STIXNonUnicode, STIXSizeOneSym, STIXGeneral, Standard Symbols L, DejaVu Sans
geo.wifi.urihttps://www.googleapis.com/geolocation/v1/geolocate?key=%GOOGLE_API_KEY%https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%
javascript.options.typeinferencetrueundefined
layers.use-deprecated-texturestrueundefined
layout.css.variables.enabledfalsetrue
media.navigator.video.default_height4800
media.navigator.video.default_width6400
media.peerconnection.aec1undefined
media.peerconnection.aec_enabledtrueundefined
media.peerconnection.agc1undefined
media.peerconnection.agc_enabledfalseundefined
media.peerconnection.identity.timeout500010000
media.peerconnection.noise1undefined
media.peerconnection.noise_enabledfalseundefined
media.webvtt.enabledfalsetrue
memory_info_dumper.watch_fifofalseundefined
services.sync.prefs.sync.browser.tabs.closeButtonstrueundefined
svg.paint-order.enabledfalsetrue
toolkit.telemetry.enabledfalseundefined