This is difference between the default preference of Firefox 12-beta1 and one of 11.0.

Default pref Javascript files:
(omni.ja){/greprefs.js,/defaults/pref/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js}}
PrefFirefox 11.0Firefox 12 beta 1
app.support.baseURLhttp://support.mozilla.com/1/firefox/%VERSION%/%OS%/%LOCALE%/http://support.mozilla.org/1/firefox/%VERSION%/%OS%/%LOCALE%/
browser.newtab.urlundefinedabout:blank
browser.newtabpage.enabledundefinedfalse
browser.sessionstore.restore_pinned_tabs_on_demandundefinedfalse
browser.tabs.onTopundefinedtrue
capability.policy.default.History.currentUniversalBrowserReadUniversalXPConnect
capability.policy.default.History.itemUniversalBrowserReadUniversalXPConnect
capability.policy.default.History.nextUniversalBrowserReadUniversalXPConnect
capability.policy.default.History.previousUniversalBrowserReadUniversalXPConnect
capability.policy.default.History.toStringUniversalBrowserReadUniversalXPConnect
devtools.styleeditor.transitionsundefinedtrue
devtools.tilt.intro_transitionundefinedtrue
devtools.tilt.outro_transitionundefinedtrue
dom.network.enabledundefinedtrue
dom.network.meteredundefinedfalse
extensions.webservice.discoverURLhttps://services.addons.mozilla.org/%LOCALE%/firefox/discovery/pane/%VERSION%/%OS%https://services.addons.mozilla.org/%LOCALE%/firefox/discovery/pane/%VERSION%/%OS%/%COMPATIBILITY_MODE%
intl.charsetmenu.browser.more4armscii-8, GEOSTD8, TIS-620, ISO-8859-11, windows-874, IBM857, ISO-8859-9, x-mac-turkish, windows-1254, x-viet-tcvn5712, VISCII, x-viet-vps, windows-1258, x-mac-devanagari, x-mac-gujarati, x-mac-gurmukhiarmscii-8, TIS-620, ISO-8859-11, windows-874, IBM857, ISO-8859-9, x-mac-turkish, windows-1254, x-viet-tcvn5712, VISCII, x-viet-vps, windows-1258, x-mac-devanagari, x-mac-gujarati, x-mac-gurmukhi
services.sync.prefs.sync.browser.tabs.onTopundefinedtrue