This is difference between the default preference of Firefox 4.0.1 and one of 5.0b2. Unlike 3.6 and 4.0, only a few of preferences are changed.

Default pref Javascript files:
(omni.jar){/greprefs.js,/defaults/pref/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js}}
4.0.15.0b2
accessibility.typeaheadfind.enablesoundtruefalse
browser.panorama.welcome_urlhttp://www.mozilla.com/firefox/panorama/undefined
custtoolbar.personal_toolbar_folderundefined
dom.min_background_timeout_valueundefined1000
dom.min_timeout_value104
general.startup.browsertrueundefined
gfx.font_rendering.harfbuzz.level2undefined
gfx.font_rendering.harfbuzz.scriptsundefined3
intl.charsetmenu.browser.more3GB2312, x-gbk, gb18030, HZ-GB-2312, ISO-2022-CN, Big5, Big5-HKSCS, x-euc-tw, EUC-JP, ISO-2022-JP, Shift_JIS, EUC-KR, x-windows-949, x-johab, ISO-2022-KRGB2312, gbk, gb18030, HZ-GB-2312, ISO-2022-CN, Big5, Big5-HKSCS, x-euc-tw, EUC-JP, ISO-2022-JP, Shift_JIS, EUC-KR, x-windows-949, x-johab, ISO-2022-KR
intl.charsetmenu.browser.unicodeUTF-8, UTF-16LE, UTF-16BE, UTF-32, UTF-32LE, UTF-32BEUTF-8, UTF-16LE, UTF-16BE
profile.allow_automigrationfalseundefined
services.sync.sendVersionInfoundefinedtrue
services.sync.storageAPI1.0undefined