This is difference between the default preference of Firefox 26.0 beta 5 and one of 25.0.

Default pref Javascript files (Firefox 25.0 for Linux):
(omni.ja){/greprefs.js,/defaults/pref/services-sync.js}, (browser/omni.ja)defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js}
Default pref Javascript files (Firefox 26.0 beta 5 for Linux):
(omni.ja){/greprefs.js,/defaults/pref/services-sync.js}, (browser/omni.ja)defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js}
PrefFirefox 25.0Firefox 26.0 beta 5
browser.download.useJSTransferfalseundefined
browser.download.useToolkitUIfalseundefined
browser.helperApps.deleteTempFileOnExitundefinedtrue
browser.mixedcontent.warning.infoURLhttp://support.mozilla.org/1/%APP%/%VERSION%/%OS%/%LOCALE%/mixed-content/undefined
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.appmanager.enabledundefinedtrue
devtools.appmanager.firstrunundefinedtrue
devtools.debugger.enable-content-actorsfalsetrue
devtools.debugger.forbid-certified-appsundefinedtrue
devtools.debugger.ignore-caught-exceptionsundefinedtrue
devtools.debugger.prompt-connectionundefinedtrue
devtools.inspector.show_pseudo_elementsundefinedtrue
devtools.layoutview.enabledtrueundefined
devtools.layoutview.openfalseundefined
devtools.responsiveUI.no-reload-notificationundefinedfalse
devtools.toolbox.zoomValueundefined1
dom.cellbroadcast.enabledundefinedfalse
dom.experimental_forms_rangetrueundefined
dom.forms.numberundefinedfalse
dom.icc.enabledundefinedfalse
dom.indexedDB.experimentalundefinedfalse
dom.inter-app-communication-api.enabledundefinedfalse
dom.mms.requestStatusReportundefinedtrue
dom.mms.versionundefined19
dom.mobileconnection.enabledundefinedfalse
dom.mozInputMethod.enabledundefinedfalse
dom.omit_components_in_contenttrueundefined
dom.quotaManager.testingundefinedfalse
dom.telephony.enabledundefinedfalse
dom.voicemail.enabledundefinedfalse
gfx.font_rendering.opentype_svg.enabledfalsetrue
gfx.font_rendering.wordcache.charlimitundefined32
gfx.font_rendering.wordcache.maxentriesundefined10000
javascript.options.ion.chromeundefinedfalse
javascript.options.mem.analysis_purge_mb100undefined
javascript.options.mem.disable_explicit_compartment_gctrueundefined
javascript.options.parallel_parsingundefinedtrue
javascript.options.typeinferencetrueundefined
javascript.options.typeinference.chromeundefinedfalse
javascript.options.typeinference.contentundefinedtrue
layers.componentalpha.enabledundefinedtrue
layers.draw-bigimage-bordersundefinedfalse
layers.draw-tile-bordersundefinedfalse
layers.max-activeundefined-1
layers.offmainthreadcomposition.force-basicundefinedfalse
layout.css.image-orientation.enabledundefinedtrue
layout.css.mix-blend-mode.enabledundefinedfalse
layout.css.sticky.enabledundefinedfalse
layout.imagevisibility.enabledundefinedtrue
layout.imagevisibility.numscrollportheightsundefined1
layout.imagevisibility.numscrollportwidthsundefined0
media.gstreamer.enabledfalsetrue
network.warnOnAboutNetworkingundefinedtrue
nglayout.events.dispatchLeftClickOnlytrueundefined
offline-apps.allow_by_defaultundefinedtrue
plugin.default.state21
plugin.state.flashundefined2
security.ssl.enable_ocsp_staplingfalsetrue
security.tls.version.max12
services.sync.engine.appsfalseundefined
signon.useDOMFormHasPasswordundefinedtrue