This is difference between the default preference of Firefox 16.0 beta3 and one of 15.0.1.

Default pref Javascript files (Firefox 15.0.1 for linux):
(omni.ja){/greprefs.js,/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js,services-aitc.js}}
Default pref Javascript files (Firefox 16.0 beta3 for linux):
(omni.ja){/greprefs.js,/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js,services-aitc.js}}
PrefFirefox 15.0.1Firefox 16.0 beta 3
accessibility.force_disabledundefined0
accessibility.win32.force_disabledfalseundefined
devtools.commands.dirundefined
devtools.scratchpad.recentFilesMaxundefined10
devtools.toolbar.enabledfalsetrue
devtools.toolbar.visibleundefinedfalse
devtools.webconsole.positionabovebelow
dom.experimental_formsundefinedfalse
dom.idle-observers-api.enabledundefinedtrue
dom.mozAlarms.enabledundefinedfalse
dom.mozApps.usedundefinedfalse
javascript.options.mem.gc_dynamic_heap_growthundefinedtrue
javascript.options.mem.gc_dynamic_mark_sliceundefinedtrue
javascript.options.mem.gc_high_frequency_heap_growth_maxundefined300
javascript.options.mem.gc_high_frequency_heap_growth_minundefined150
javascript.options.mem.gc_high_frequency_high_limit_mbundefined500
javascript.options.mem.gc_high_frequency_low_limit_mbundefined100
javascript.options.mem.gc_high_frequency_time_limit_msundefined1000
javascript.options.mem.gc_incrementalfalsetrue
javascript.options.mem.gc_low_frequency_heap_growthundefined150
javascript.options.mem.notifyundefinedfalse
javascript.options.xml.chromeundefinedtrue
javascript.options.xml.contentundefinedtrue
layers.async-video.enabledundefinedfalse
network.http.connection-timeoutundefined90
network.http.spdy.enabled.v3falsetrue
network.websocket.delay-failed-reconnectsundefinedtrue
security.enable_md5_signaturestruefalse
services.aitc.browserid.urlhttps://browserid.org/sign_inhttps://login.persona.org/sign_in
services.aitc.log.dumpundefinedfalse
services.aitc.log.levelundefinedAll
social.enabledundefinedfalse
social.manifest.motownundefined{"origin":"https://motown-dev.mozillalabs.com","name":"MoTown","workerURL":"https://motown-dev.mozillalabs.com/social/worker.js","iconURL":"https://motown-dev.mozillalabs.com/images/motown-icon.png"}
toolkit.identity.debugundefinedfalse