This is difference between the default preference of Firefox 7.0 and one of 6.0.

Default pref Javascript files:
(omni.jar){/greprefs.js,/defaults/pref/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js}}
Firefox 6Firefox 7
app.update.nagTimer.restart86400undefined
browser.cache.disk.max_entry_sizeundefined5120
browser.cache.memory.max_entry_sizeundefined5120
browser.urlbar.trimURLsundefinedtrue
devtools.editor.expandtabundefinedtrue
devtools.editor.tabsizeundefined4
dom.enable_performanceundefinedtrue
extensions.blocklist.itemURLundefinedhttps://addons.mozilla.org/%LOCALE%/%APP%/blocked/%blockID%
font.mathfont-familySTIXNonUnicode, STIXSizeOneSym, STIXSize1, STIXGeneral, Standard Symbols L, DejaVu Sans, Cambria MathSTIXNonUnicode, STIXSizeOneSym, STIXSize1, STIXGeneral, Asana Math, Standard Symbols L, DejaVu Sans, Cambria Math
image.mem.min_discard_timeout_ms12000010000
javascript.options.mem.gc_frequency300undefined
network.websocket.extensions.stream-deflatefalsetrue
network.websocket.max-connectionsundefined200
services.sync.autoconnecttrueundefined
services.sync.log.appender.debugLogTraceundefined
services.sync.log.appender.debugLog.enabledfalseundefined
services.sync.log.appender.debugLog.maxSize1048576undefined
services.sync.log.appender.debugLog.rotatetrueundefined
services.sync.log.appender.file.levelundefinedTrace
services.sync.log.appender.file.logOnErrorundefinedtrue
services.sync.log.appender.file.logOnSuccessundefinedfalse
services.sync.scheduler.activeIntervalundefined300
services.sync.scheduler.idleIntervalundefined3600
services.sync.scheduler.idleTimeundefined300
services.sync.scheduler.immediateIntervalundefined60
services.sync.scheduler.singleDeviceIntervalundefined3600
toolkit.telemetry.infoURLundefinedhttp://www.mozilla.com/legal/privacy/firefox.html#telemetry
toolkit.telemetry.serverhttp://telemetry.allizom.orghttps://data.mozilla.com
toolkit.telemetry.server_ownerundefinedMozilla
ui.use_native_popup_windowsfalseundefined