This is difference between the default preference of Firefox 20 beta4 and one of 19.0.2.

Default pref Javascript files (Firefox 19.0.2 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 20 beta 4 for linux):
(omni.ja){/greprefs.js,/defaults/pref/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js,services-aitc.js}}
PrefFirefox 19.0.2Firefox 20 beta 4
browser.download.panel.removeFinishedDownloadsfalseundefined
browser.download.useToolkitUItruefalse
browser.privatebrowsing.dont_prompt_on_enterfalseundefined
browser.warnOnRestartfalseundefined
datareporting.policy.dataSubmissionEnabledundefinedtrue
datareporting.policy.dataSubmissionPolicyAcceptedundefinedfalse
datareporting.policy.dataSubmissionPolicyBypassAcceptanceundefinedfalse
datareporting.policy.dataSubmissionPolicyNotifiedTimeundefined0
datareporting.policy.dataSubmissionPolicyResponseTimeundefined0
datareporting.policy.dataSubmissionPolicyResponseTypeundefined
datareporting.policy.firstRunTimeundefined0
devtools.debugger.ui.height250undefined
devtools.debugger.ui.variables-non-enum-visibletrueundefined
devtools.debugger.ui.variables-only-enum-visibleundefinedfalse
devtools.hud.height0undefined
devtools.inspector.htmlHeight112undefined
devtools.inspector.htmlPanelOpenfalseundefined
devtools.inspector.sidebarOpenfalseundefined
devtools.profiler.enabledundefinedfalse
devtools.ruleview.enabledtrueundefined
devtools.styleinspector.enabledtrueundefined
devtools.toolbox.footer.heightundefined250
devtools.toolbox.hostundefinedbottom
devtools.toolbox.selectedToolundefinedwebconsole
devtools.toolbox.sideEnabledundefinedfalse
devtools.toolbox.sidebar.widthundefined500
devtools.toolbox.toolbarSpecundefined["tilt toggle","scratchpad","resize toggle"]
devtools.webconsole.positionbelowundefined
intl.charsetmenu.browser.unicodeUTF-8, UTF-16LE, UTF-16BEundefined
javascript.options.ion.parallel_compilationundefinedtrue
javascript.options.xml.chrometruefalse
layers.force-activeundefinedfalse
layout.css.masking.enabledundefinedfalse
layout.css.scope-pseudo.enabledundefinedfalse
media.dash.enabledundefinedfalse
media.navigator.enabledfalsetrue
mousewheel.default.action.override_xundefined-1
mousewheel.with_alt.action.override_xundefined-1
mousewheel.with_control.action.override_xundefined-1
mousewheel.with_meta.action.override_xundefined-1
mousewheel.with_shift.action.override_xundefined-1
mousewheel.with_win.action.override_xundefined-1
network.http.rendering-critical-requests-prioritizationundefinedtrue
network.http.speculative-parallel-limitundefined6
social.activefalseundefined
toolkit.crashreporter.infoURLundefinedhttp://www.mozilla.com/legal/privacy/firefox.html#crash-reporter
wap.UAProf.tagnameundefinedx-wap-profile
wap.UAProf.urlundefined