This is difference between the default preference of Firefox 67.0 beta 8 and one of 66.0.2.

Preference difference
PrefFirefox 66.0.2Firefox 67.0 beta 8
app.normandy.remotesettings.enabledfalseundefined
app.releaseNotesURLhttps://www.mozilla.org/%LOCALE%/firefox/%VERSION%/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnewhttps://www.mozilla.org/%LOCALE%/firefox/%VERSION%beta/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnew
app.update.log.fileundefinedfalse
app.update.staging.enabledfalsetrue
app.update.url.detailshttps://www.mozilla.org/%LOCALE%/firefox/noteshttps://www.mozilla.org/%LOCALE%/firefox/beta/notes
app.update.url.manualhttps://www.mozilla.org/firefox/https://www.mozilla.org/firefox/beta
apz.pinch_lock.buffer_max_ageundefined50
browser.aboutConfig.showWarningundefinedtrue
browser.cache.disk.smart_size.first_runtrueundefined
browser.cache.disk.smart_size.use_old_maxtrueundefined
browser.contentblocking.cryptomining.preferences.ui.enabledundefinedtrue
browser.contentblocking.fingerprinting.preferences.ui.enabledundefinedtrue
browser.contentblocking.reportBreakage.enabledfalsetrue
browser.dictionaries.download.urlhttps://addons.mozilla.org/%LOCALE%/firefox/dictionaries/https://addons.mozilla.org/%LOCALE%/firefox/language-tools/
browser.display.use_document_fonts1undefined
browser.fission.simulatefalseundefined
browser.newtabpage.activity-stream.asrouter.providers.cfr{"id":"cfr","enabled":true,"type":"local","localProvider":"CFRMessageProvider","frequency":{"custom":[{"period":"daily","cap":1}]}}{"id":"cfr","enabled":true,"type":"local","localProvider":"CFRMessageProvider","frequency":{"custom":[{"period":"daily","cap":1}]},"categories":["cfrAddons","cfrFeatures"]}
browser.safebrowsing.blockedURIs.enabledtrueundefined
browser.safebrowsing.malware.enabledtrueundefined
browser.safebrowsing.phishing.enabledtrueundefined
browser.sessionstore.dom_storage_limit2048undefined
browser.sessionstore.max_tabs_undo1025
browser.tabs.remote.useCrossOriginOpenerPolicyundefinedfalse
browser.tabs.remote.useHTTPResponseProcessSelectionundefinedtrue
browser.tabs.unloadOnLowMemoryundefinedtrue
browser.toolbars.keyboard_navigationundefinedtrue
devtools.browserconsole.ui.filterbarfalseundefined
devtools.canvasdebugger.enabledfalseundefined
devtools.debugger.features.column-breakpointsfalsetrue
devtools.debugger.features.log-pointsfalsetrue
devtools.debugger.features.pause-pointstrueundefined
devtools.debugger.features.windowless-workersfalsetrue
devtools.debugger.map-scopes-enabledundefinedfalse
devtools.debugger.ui.editor-wrappingundefinedfalse
devtools.flexboxinspector.enabledtrueundefined
devtools.inspector.changes.enabledtrueundefined
devtools.inspector.flexboxHighlighter.enabledtrueundefined
devtools.inspector.fonthighlighter.enabledtrueundefined
devtools.inspector.scrollable-badges.enabledfalseundefined
devtools.inspector.shapesHighlighter.enabledtrueundefined
devtools.layout.boxmodel.highlightPropertyundefinedfalse
devtools.markup.beautifyOnCopyundefinedfalse
devtools.netmonitor.columnsDataundefined[{"name":"status","minWidth":30,"width":5}, {"name":"method","minWidth":30,"width":5}, {"name":"domain","minWidth":30,"width":10}, {"name":"file","minWidth":30,"width":25}, {"name":"cause","minWidth":30,"width":10},{"name":"type","minWidth":30,"width":5},{"name":"transferred","minWidth":30,"width":10},{"name":"contentSize","minWidth":30,"width":5},{"name":"waterfall","minWidth":150,"width":25}]
devtools.netmonitor.features.resizeColumnsundefinedfalse
devtools.netmonitor.response.ui.limitundefined10240
devtools.performance.recording.objdirsundefined[]
devtools.performance.recording.ui-base-urlhttps://perf-html.iohttps://profiler.firefox.com
devtools.shadereditor.enabledfalseundefined
devtools.webaudioeditor.enabledfalseundefined
devtools.webaudioeditor.inspectorWidth300undefined
devtools.webconsole.groupWarningMessagesundefinedfalse
devtools.webconsole.ui.filterbarfalseundefined
dom.event.highrestimestamp.enabledtrueundefined
dom.idle-observers-api.fuzz_time.disabledtrueundefined
dom.inputevent.data.enabledundefinedtrue
dom.inputevent.datatransfer.enabledundefinedtrue
dom.keyboardevent.keypress.hack.use_legacy_keycode_and_charcode*.gov.online.office365.us,*.officeapps-df.live.com.,*.officeapps.live.com,*.online.office.de,*.partner.officewebapps.cn,mail.notes.na.collabserv.com*.collabserv.com,*.gov.online.office365.us,*.officeapps-df.live.com,*.officeapps.live.com,*.online.office.de,*.partner.officewebapps.cn,*.scniris.com
dom.popup_allowed_eventschange click dblclick mouseup pointerup notificationclick reset submit touchendchange click dblclick mouseup pointerup notificationclick reset submit touchend contextmenu
dom.push.alwaysConnectfalseundefined
dom.select_popup_in_content.enabledfalseundefined
dom.storage.client_validationundefinedtrue
dom.storage.next_genfalsetrue
dom.ua_widget.enabledtrueundefined
extensions.geckoProfiler.getSymbolRuleslocalBreakpad,nmundefined
extensions.htmlaboutaddons.enabledundefinedfalse
extensions.webcompat-reporter.enabledfalsetrue
extensions.webextensions.restrictedDomainsaccounts-static.cdn.mozilla.net,accounts.firefox.com,addons.cdn.mozilla.net,addons.mozilla.org,api.accounts.firefox.com,content.cdn.mozilla.net,discovery.addons.mozilla.org,input.mozilla.org,install.mozilla.org,oauth.accounts.firefox.com,profile.accounts.firefox.com,support.mozilla.org,sync.services.mozilla.com,testpilot.firefox.comaccounts-static.cdn.mozilla.net,accounts.firefox.com,addons.cdn.mozilla.net,addons.mozilla.org,api.accounts.firefox.com,content.cdn.mozilla.net,discovery.addons.mozilla.org,input.mozilla.org,install.mozilla.org,oauth.accounts.firefox.com,profile.accounts.firefox.com,support.mozilla.org,sync.services.mozilla.com
features.normandy-remote-settings.enabledundefinedfalse
fission.frontend.simulate-eventsundefinedfalse
fission.frontend.simulate-messagesundefinedfalse
font.name-list.cursive.arundefinedcursive
font.name-list.cursive.elundefinedcursive
font.name-list.cursive.heundefinedcursive
font.name-list.cursive.jaundefinedcursive
font.name-list.cursive.koundefinedcursive
font.name-list.cursive.thundefinedcursive
font.name-list.cursive.x-armnundefinedcursive
font.name-list.cursive.x-bengundefinedcursive
font.name-list.cursive.x-cansundefinedcursive
font.name-list.cursive.x-cyrillicundefinedcursive
font.name-list.cursive.x-devanagariundefinedcursive
font.name-list.cursive.x-ethiundefinedcursive
font.name-list.cursive.x-georundefinedcursive
font.name-list.cursive.x-gujrundefinedcursive
font.name-list.cursive.x-guruundefinedcursive
font.name-list.cursive.x-khmrundefinedcursive
font.name-list.cursive.x-kndaundefinedcursive
font.name-list.cursive.x-mlymundefinedcursive
font.name-list.cursive.x-oryaundefinedcursive
font.name-list.cursive.x-sinhundefinedcursive
font.name-list.cursive.x-tamilundefinedcursive
font.name-list.cursive.x-teluundefinedcursive
font.name-list.cursive.x-tibtundefinedcursive
font.name-list.cursive.x-unicodeundefinedcursive
font.name-list.cursive.x-westernundefinedcursive
font.name-list.cursive.zh-CNundefinedcursive
font.name-list.cursive.zh-HKundefinedcursive
font.name-list.cursive.zh-TWundefinedcursive
geo.wifi.urihttps://www.googleapis.com/geolocation/v1/geolocate?key=%GOOGLE_LOCATION_SERVICE_API_KEY%https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%
gfx.canvas.skiagl.dynamic-cachetrueundefined
gfx.webrender.debug.small-screenundefinedfalse
gl.allow-high-powerundefinedtrue
identity.fxaccounts.pairing.enabledundefinedtrue
identity.fxaccounts.remote.pairing.uriundefinedwss://channelserver.services.mozilla.com
identity.fxaccounts.toolbar.accessedundefinedfalse
identity.fxaccounts.toolbar.enabledundefinedtrue
javascript.options.dynamicImportfalsetrue
javascript.options.experimental.fieldsundefinedfalse
media.av1.enabledfalsetrue
media.av1.use-dav1dfalsetrue
media.gmp-gmpopenh264.enabledundefinedtrue
media.gmp-gmpopenh264.visibleundefinedtrue
network.cookie.cookieBehaviorundefined4
network.cookie.leave-secure-alonetrueundefined
network.http.referer.defaultPolicy.trackersundefined3
network.http.referer.defaultPolicy.trackers.pbmodeundefined2
network.proxy.allow_hijacking_localhostundefinedfalse
network.proxy.no_proxies_onlocalhost, 127.0.0.1
prio.enabledundefinedfalse
privacy.permissionPrompts.showCloseButtonfalseundefined
privacy.resistFingerprinting.jsmloglevelundefinedWarn
security.block_importScripts_with_wrong_mimeundefinedtrue
security.certerrors.mitm.auto_enable_enterprise_rootsundefinedfalse
security.certerrors.mitm.priming.enabledundefinedtrue
security.certerrors.mitm.priming.endpointundefinedhttps://mitmdetection.services.mozilla.com/
security.remote_settings.intermediates.downloads_per_pollundefined100
services.common.uptake.sampleRateundefined1
services.sync.engine.bookmarks.validation.enabledundefinedtrue
services.sync.engine.passwords.validation.enabledundefinedtrue
services.sync.prefs.sync.media.autoplay.defaultundefinedtrue
services.sync.prefs.sync.privacy.trackingprotection.cryptomining.annotate.enabledundefinedtrue
services.sync.prefs.sync.privacy.trackingprotection.cryptomining.enabledundefinedtrue
services.sync.prefs.sync.privacy.trackingprotection.fingerprinting.annotate.enabledundefinedtrue
services.sync.prefs.sync.privacy.trackingprotection.fingerprinting.enabledundefinedtrue
signon.autofillForms.autocompleteOffundefinedtrue
signon.privateBrowsingCapture.enabledundefinedtrue
signon.showAutoCompleteFooterundefinedtrue
toolkit.lazyHiddenWindowundefinedtrue
urlclassifier.disallow_completionstest-malware-simple,test-harmful-simple,test-phish-simple,test-unwanted-simple,test-track-simple,test-trackwhite-simple,test-block-simple,goog-downloadwhite-digest256,base-track-digest256,mozstd-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,block-flash-digest256,except-flash-digest256,allow-flashallow-digest256,except-flashallow-digest256,block-flashsubdoc-digest256,except-flashsubdoc-digest256,goog-passwordwhite-proto,ads-track-digest256,social-track-digest256,analytics-track-digest256,base-fingerprinting-track-digest256,content-fingerprinting-track-digest256,base-cryptomining-track-digest256,content-cryptomining-track-digest256,fanboyannoyance-ads-digest256,fanboysocial-ads-digest256,easylist-ads-digest256,easyprivacy-ads-digest256,adguard-ads-digest256goog-downloadwhite-digest256,base-track-digest256,mozstd-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,block-flash-digest256,except-flash-digest256,allow-flashallow-digest256,except-flashallow-digest256,block-flashsubdoc-digest256,except-flashsubdoc-digest256,goog-passwordwhite-proto,ads-track-digest256,social-track-digest256,analytics-track-digest256,base-fingerprinting-track-digest256,content-fingerprinting-track-digest256,base-cryptomining-track-digest256,content-cryptomining-track-digest256,fanboyannoyance-ads-digest256,fanboysocial-ads-digest256,easylist-ads-digest256,easyprivacy-ads-digest256,adguard-ads-digest256
urlclassifier.trackingAnnotationTabletest-track-simple,base-track-digest256test-track-simple,base-track-digest256,content-track-digest256
webgl.default-low-powerundefinedfalse
Default pref Javascript files
PrefFirefox 66.0.2Firefox 67.0 beta 8
browser/omni.ja.d/defaults/preferences/debugger.jspresentpresent
browser/omni.ja.d/defaults/preferences/devtools-client.jspresentpresent
browser/omni.ja.d/defaults/preferences/devtools-startup.jspresentpresent
browser/omni.ja.d/defaults/preferences/firefox-branding.jspresentpresent
browser/omni.ja.d/defaults/preferences/firefox-l10n.jspresentpresent
browser/omni.ja.d/defaults/preferences/firefox.jspresentpresent
browser/omni.ja.d/defaults/preferences/webide.jspresentpresent
omni.ja.d/defaults/pref/marionette.jspresentpresent
omni.ja.d/defaults/pref/services-sync.jspresentpresent
omni.ja.d/greprefs.jspresentpresent