This is difference between the default preference of Firefox 18 beta3 and one of 17.0.1.

Default pref Javascript files (Firefox 17.0.1 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 18 beta3 for linux):
(omni.ja){/greprefs.js,/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js,services-aitc.js}}
PrefFirefox 17.0.1Firefox 18 beta 3
browser.zoom.reflowOnZoomundefinedfalse
devtools.debugger.remote-timeout300020000
devtools.inspector.markupPreviewundefinedfalse
dom.browserElement.maxScreenshotDelayMSundefined2000
dom.identity.enabledundefinedfalse
dom.mozApps.dev_modeundefinedfalse
dom.mozApps.maxLocalIdundefined1000
dom.mozNetworkStats.enabledundefinedfalse
dom.mozPermissionSettings.enabledundefinedfalse
dom.sms.strict7BitEncodingundefinedfalse
extensions.getMoreThemesURLhttps://addons.mozilla.org/%LOCALE%/firefox/getpersonasundefined
font.size.inflation.disabledInMasterProcessundefinedfalse
font.size.inflation.forceEnabledundefinedfalse
gfx.canvas.azure.enabledfalsetrue
gfx.content.azure.backendsundefinedcairo
gfx.font_rendering.opentype_svg.enabledundefinedfalse
image.high_quality_downscaling.enabledundefinedfalse
image.high_quality_downscaling.min_factorundefined1000
javascript.options.ion.contentundefinedtrue
javascript.options.mem.gc_allocation_threshold_mbundefined30
jsloader.reuseGlobalundefinedfalse
layers.composer2d.enabledundefinedfalse
layout.css.devPixelsPerPx1.0-1.0
layout.css.flexbox.enabledundefinedfalse
media.navigator.permission.disabledundefinedfalse
media.peerconnection.enabledundefinedfalse
media.video-queue.default-sizeundefined10
media.webaudio.enabledundefinedfalse
memory.free_dirty_pagesundefinedfalse
network.activity.blipIntervalMillisecondsundefined0
network.dns.disablePrefetchundefinedfalse
network.dnsCacheExpirationGracePeriodundefined2592000
network.http.max-persistent-connections-per-proxy832
network.http.pipelining.abtestundefinedfalse
offline-apps.quota.max204800undefined
security.mixed_content.block_active_contentundefinedfalse
security.mixed_content.block_display_contentundefinedfalse
toolkit.identity.enabledundefinedfalse
ui.mouse.radius.bottommmundefined4
ui.mouse.radius.enabledundefinedfalse
ui.mouse.radius.leftmmundefined8
ui.mouse.radius.rightmmundefined8
ui.mouse.radius.topmmundefined12
ui.mouse.radius.visitedWeightundefined120
ui.touch.radius.bottommmundefined4
ui.touch.radius.enabledundefinedfalse
ui.touch.radius.leftmmundefined8
ui.touch.radius.rightmmundefined8
ui.touch.radius.topmmundefined12
ui.touch.radius.visitedWeightundefined120
webgl.force_osmesafalseundefined
webgl.osmesalibundefined