This is difference between the default preference of Firefox 5.0 and one of 6.0b2.

Default pref Javascript files:
(omni.jar){/greprefs.js,/defaults/pref/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js}}
Firefox 5Firefox 6
accelerometer.enabledtrueundefined
app.support.baseURLhttp://support.mozilla.com/1/%APP%/%VERSION%/%OS%/%LOCALE%/http://support.mozilla.com/1/firefox/%VERSION%/%OS%/%LOCALE%/
app.update.timer600000undefined
app.update.timerMinimumDelayundefined120
bidi.characterset1undefined
browser.dictionaries.download.urlhttps://addons.mozilla.org/%LOCALE%/%APP%/dictionaries/https://addons.mozilla.org/%LOCALE%/firefox/dictionaries/
browser.geolocation.warning.infoURLhttp://www.mozilla.com/%LOCALE%/%APP%/geolocation/http://www.mozilla.com/%LOCALE%/firefox/geolocation/
browser.microsummary.enabledtrueundefined
browser.microsummary.updateGeneratorstrueundefined
browser.safebrowsing.warning.infoURLhttp://www.mozilla.com/%LOCALE%/%APP%/phishing-protection/http://www.mozilla.com/%LOCALE%/firefox/phishing-protection/
browser.search.searchEnginesURLhttps://addons.mozilla.org/%LOCALE%/%APP%/search-engines/https://addons.mozilla.org/%LOCALE%/firefox/search-engines/
browser.sessionhistory.optimize_evictionfalsetrue
browser.sessionstore.restore_hidden_tabsundefinedfalse
browser.urlbar.formatting.enabledundefinedtrue
device.motion.enabledundefinedtrue
devtools.chrome.enabledundefinedfalse
devtools.hud.loglimit.consoleundefined200
devtools.hud.loglimit.cssparserundefined200
devtools.hud.loglimit.exceptionundefined200
devtools.hud.loglimit.networkundefined200
devtools.scratchpad.enabledundefinedtrue
devtools.webconsole.positionundefinedabove
dom.server-events.default-reconnection-timeundefined5000
dom.server-events.enabledundefinedtrue
extensions.getAddons.get.urlhttps://services.addons.mozilla.org/%LOCALE%/%APP%/api/%API_VERSION%/search/guid:%IDS%?src=firefox&appOS=%OS%&appVersion=%VERSION%&tMain=%TIME_MAIN%&tFirstPaint=%TIME_FIRST_PAINT%&tSessionRestored=%TIME_SESSION_RESTORED%https://services.addons.mozilla.org/%LOCALE%/firefox/api/%API_VERSION%/search/guid:%IDS%?src=firefox&appOS=%OS%&appVersion=%VERSION%&tMain=%TIME_MAIN%&tFirstPaint=%TIME_FIRST_PAINT%&tSessionRestored=%TIME_SESSION_RESTORED%
extensions.getAddons.search.browseURLhttps://addons.mozilla.org/%LOCALE%/%APP%/search?q=%TERMS%https://addons.mozilla.org/%LOCALE%/firefox/search?q=%TERMS%
extensions.getAddons.search.urlhttps://services.addons.mozilla.org/%LOCALE%/%APP%/api/%API_VERSION%/search/%TERMS%/all/%MAX_RESULTS%/%OS%/%VERSION%?src=firefoxhttps://services.addons.mozilla.org/%LOCALE%/firefox/api/%API_VERSION%/search/%TERMS%/all/%MAX_RESULTS%/%OS%/%VERSION%?src=firefox
extensions.getMoreThemesURLhttps://addons.mozilla.org/%LOCALE%/%APP%/getpersonashttps://addons.mozilla.org/%LOCALE%/firefox/getpersonas
extensions.webservice.discoverURLhttps://services.addons.mozilla.org/%LOCALE%/%APP%/discovery/pane/%VERSION%/%OS%https://services.addons.mozilla.org/%LOCALE%/firefox/discovery/pane/%VERSION%/%OS%
gfx.downloadable_fonts.sanitize.preserve_otl_tablestrueundefined
intl.accept_charsetsiso-8859-1,*,utf-8undefined
intl.hyphenation-alias.enundefineden-us
intl.hyphenation-alias.en-*undefineden-us
javascript.options.jitprofiling.chromefalsetrue
javascript.options.methodjit.chromefalsetrue
media.volume_scaleundefined1.0
mousewheel.enable_pixel_scrollingtrueundefined
network.IDN.whitelist.asiaundefinedtrue
network.IDN.whitelist.lvundefinedtrue
network.IDN.whitelist.xn--fzc2c9e2cundefinedtrue
network.IDN.whitelist.xn--ogbpf8flundefinedtrue
network.IDN.whitelist.xn--xkc2al3hye2aundefinedtrue
network.http.connection-retry-timeoutundefined250
network.http.max-connections30256
network.websocket.auto-follow-http-redirectsundefinedfalse
network.websocket.extensions.stream-deflateundefinedfalse
network.websocket.max-message-sizeundefined16000000
network.websocket.override-security-blockfalseundefined
network.websocket.timeout.closeundefined20
network.websocket.timeout.openundefined20
network.websocket.timeout.ping.requestundefined0
network.websocket.timeout.ping.responseundefined10
toolkit.telemetry.enabledundefinedfalse
toolkit.telemetry.serverundefinedhttp://telemetry.allizom.org