This is difference between the default preference of Firefox 13-beta1 and one of 12.0.

Default pref Javascript files:
(omni.ja){/greprefs.js,/defaults/pref/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js}}
PrefFirefox 12.0Firefox 13 beta 1
browser.altClickSaveundefinedfalse
browser.newtab.urlabout:blankabout:newtab
browser.newtabpage.enabledfalsetrue
browser.search.context.loadInBackgroundundefinedfalse
browser.sessionstore.restore_on_demandfalsetrue
browser.urlbar.autoFill.typedundefinedtrue
devtools.debugger.enabledundefinedfalse
devtools.debugger.logundefinedfalse
devtools.debugger.ui.heightundefined250
devtools.inspector.activeSidebarundefinedruleview
devtools.inspector.htmlPanelOpenundefinedfalse
devtools.inspector.sidebarOpenundefinedfalse
dom.event.clipboardevents.enabledundefinedtrue
dom.mozContacts.enabledundefinedfalse
dom.mozContacts.whitelistundefined
editor.quotesPreformattedfalseundefined
extensions.getAddons.search.browseURLhttps://addons.mozilla.org/%LOCALE%/firefox/search?q=%TERMS%https://addons.mozilla.org/%LOCALE%/firefox/search?q=%TERMS%&platform=%OS%&appver=%VERSION%
extensions.update.background.urlundefinedhttps://versioncheck-bg.addons.mozilla.org/update/VersionCheck.php?reqVersion=%REQ_VERSION%&id=%ITEM_ID%&version=%ITEM_VERSION%&maxAppVersion=%ITEM_MAXAPPVERSION%&status=%ITEM_STATUS%&appID=%APP_ID%&appVersion=%APP_VERSION%&appOS=%APP_OS%&appABI=%APP_ABI%&locale=%APP_LOCALE%&currentAppVersion=%CURRENT_APP_VERSION%&updateType=%UPDATE_TYPE%&compatMode=%COMPATIBILITY_MODE%
font.mathfont-familySTIXNonUnicode, STIXSizeOneSym, STIXSize1, STIXGeneral, Asana Math, Standard Symbols L, DejaVu Sans, Cambria MathMathJax_Main, STIXNonUnicode, STIXSizeOneSym, STIXSize1, STIXGeneral, Asana Math, Standard Symbols L, DejaVu Sans, Cambria Math
full-screen-api.exit-on-deactivateundefinedtrue
general.smoothScrollfalsetrue
general.smoothScroll.durationToIntervalRatioundefined200
general.smoothScroll.linesundefinedtrue
general.smoothScroll.lines.durationMaxMSundefined150
general.smoothScroll.lines.durationMinMSundefined150
general.smoothScroll.mouseWheelundefinedtrue
general.smoothScroll.mouseWheel.durationMaxMSundefined400
general.smoothScroll.mouseWheel.durationMinMSundefined200
general.smoothScroll.otherundefinedtrue
general.smoothScroll.other.durationMaxMSundefined150
general.smoothScroll.other.durationMinMSundefined150
general.smoothScroll.pagesundefinedtrue
general.smoothScroll.pages.durationMaxMSundefined150
general.smoothScroll.pages.durationMinMSundefined150
general.smoothScroll.pixelsundefinedtrue
general.smoothScroll.pixels.durationMaxMSundefined150
general.smoothScroll.pixels.durationMinMSundefined150
general.smoothScroll.scrollbarsundefinedtrue
general.smoothScroll.scrollbars.durationMaxMSundefined150
general.smoothScroll.scrollbars.durationMinMSundefined150
gfx.font_rendering.fallback.always_use_cmapsundefinedfalse
gfx.font_rendering.harfbuzz.scripts37
gfx.show_checkerboard_patternundefinedtrue
javascript.options.mem.gc_incrementalundefinedfalse
javascript.options.mem.gc_incremental_slice_msundefined10
network.allow-experimentsundefinedtrue
network.automatic-ntlm-auth.allow-non-fqdnundefinedfalse
network.http.spdy.enabledfalsetrue
network.http.spdy.ping-thresholdundefined44
network.http.spdy.ping-timeoutundefined8
network.negotiate-auth.allow-non-fqdnundefinedfalse
nglayout.debug.paint_flashingundefinedfalse
security.enable_md5_signaturesundefinedtrue
services.sync.prefs.sync.browser.newtabpage.enabledundefinedtrue
toolkit.scrollbox.verticalScrollDistanceundefined3
toolkit.startup.max_resumed_crashesundefined2
urlclassifier.lookupcachemax1048576undefined
urlclassifier.updatecachemax41943040undefined