This is difference between the default preference of Firefox 15.0 beta2 and one of 14.0.1.

Default pref Javascript files (Firefox 14.0.1 for linux):
(omni.ja){/greprefs.js,/defaults/pref/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js}}
Default pref Javascript files (Firefox 15.0 beta2 for linux):
(omni.ja){/greprefs.js,/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js}}
PrefFirefox 14.0.1Firefox 15.0 beta 2
app.update.stage.enabledundefinedtrue
browser.identity.ssl_domain_display0undefined
browser.preferences.inContentundefinedfalse
device.storage.enabledundefinedfalse
devtools.debugger.chrome-enabledundefinedfalse
devtools.debugger.enabledfalsetrue
devtools.debugger.force-localundefinedtrue
devtools.debugger.remote-autoconnectundefinedfalse
devtools.debugger.remote-connection-retriesundefined3
devtools.debugger.remote-enabledundefinedfalse
devtools.debugger.remote-hostundefinedlocalhost
devtools.debugger.remote-portundefined6000
devtools.debugger.remote-timeoutundefined3000
devtools.debugger.ui.remote-win.heightundefined400
devtools.debugger.ui.remote-win.widthundefined900
devtools.debugger.ui.stackframes-widthundefined200
devtools.debugger.ui.variables-widthundefined300
devtools.gcli.allowSetundefinedfalse
devtools.gcli.eagerHelperundefined2
devtools.gcli.enablefalseundefined
devtools.gcli.hideIntroundefinedfalse
devtools.inspector.highlighterShowInfobarundefinedtrue
devtools.inspector.highlighterShowVeilundefinedtrue
devtools.layoutview.enabledfalsetrue
devtools.responsiveUI.enabledundefinedtrue
devtools.toolbar.enabledundefinedfalse
dom.experimental_bindingsundefinedtrue
dom.ipc.plugins.flash.subprocess.crashreporter.enabledundefinedtrue
dom.paris_bindingstrueundefined
full-screen-api.approval-requiredundefinedtrue
full-screen-api.key-input-restrictedtrueundefined
full-screen-api.warning.enabledtrueundefined
gfx.prefer-mesa-llvmpipeundefinedfalse
gfx.xrender.enabledundefinedtrue
layers.offmainthreadcomposition.enabledfalseundefined
media.opus.enabledundefinedtrue
network.http.diagnosticsundefinedfalse
network.http.spdy.enabled.v2undefinedtrue
network.http.spdy.enabled.v3undefinedfalse
pdfjs.disabledundefinedfalse
pdfjs.firstRunundefinedtrue
pdfjs.previousHandler.alwaysAskBeforeHandlingundefinedfalse
pdfjs.previousHandler.preferredActionundefined0
plugins.update.urlhttps://www.mozilla.com/%LOCALE%/plugincheck/https://www.mozilla.org/%LOCALE%/plugincheck/
services.sync.prefs.sync.browser.tabs.onToptrueundefined
toolkit.startup.max_resumed_crashes23
webgl.verbosefalseundefined