This is difference between the default preference of Firefox 78.0 beta 4 and one of 77.0.1. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 77.0.1Firefox 78.0 beta 4
app.releaseNotesURL https://www.mozilla.org/%LOCALE%/firefox/%VERSION%/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnew https://www.mozilla.org/%LOCALE%/firefox/%VERSION%beta/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnew
app.update.channel release beta
app.update.url.details https://www.mozilla.org/%LOCALE%/firefox/notes https://www.mozilla.org/%LOCALE%/firefox/beta/notes
app.update.url.manual https://www.mozilla.org/firefox/ https://www.mozilla.org/%LOCALE%/firefox/beta
apz.allow_zooming_out undefined false
apz.axis_lock.mode 0 2
apz.windows.use_direct_manipulation undefined false
browser.aboutwelcome.overrideContent undefined
browser.cache.offline.storage.enable true false
browser.contentblocking.report.endpoint_url undefined https://monitor.firefox.com/user/breach-stats?includeResolved=true
browser.contentblocking.report.monitor.home_page_url undefined https://monitor.firefox.com/user/dashboard
browser.contentblocking.report.monitor.preferences_url undefined https://monitor.firefox.com/user/preferences
browser.display.show_focus_rings true false
browser.fixup.domainsuffixwhitelist.example undefined true
browser.fixup.domainsuffixwhitelist.internal undefined true
browser.fixup.domainsuffixwhitelist.invalid undefined true
browser.fixup.domainsuffixwhitelist.local undefined true
browser.fixup.domainsuffixwhitelist.localhost undefined true
browser.fixup.domainsuffixwhitelist.test undefined true
browser.fixup.fallback-to-https undefined true
browser.helperApps.showOpenOptionForPdfJS undefined true
browser.newtabpage.activity-stream.asrouter.providers.messaging-experiments {"id":"messaging-experiments","enabled":true,"type":"remote-experiments","messageGroups":["cfr","whats-new-panel","moments-page","snippets","cfr-fxa"],"updateCycleInMs":3600000} {"id":"messaging-experiments","enabled":true,"type":"remote-experiments","messageGroups":["cfr","whats-new-panel","moments-page","snippets","cfr-fxa","aboutwelcome"],"updateCycleInMs":3600000}
browser.newtabpage.activity-stream.feeds.section.topstories false true
browser.newtabpage.activity-stream.feeds.system.topsites undefined true
browser.newtabpage.activity-stream.feeds.system.topstories undefined false
browser.newtabpage.activity-stream.improvesearch.topSiteSearchShortcuts.searchEngines google,amazon
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.region.log undefined false
browser.region.network.scan undefined false
browser.region.network.url undefined https://location.services.mozilla.com/v1/country?key=%MOZILLA_API_KEY%
browser.region.timeout undefined 5000
browser.search.modernConfig false true
browser.urlbar.maxHistoricalSearchSuggestions 0 2
browser.urlbar.openViewOnFocus true undefined
browser.urlbar.richSuggestions.tail undefined false
browser.urlbar.suggest.topsites undefined true
devtools.accessibility.auto-init.enabled undefined true
devtools.netmonitor.features.serverSentEvents undefined false
devtools.responsive.touchGestureSimulation.enabled false undefined
dom.arena_allocator.enabled false true
dom.document.exec_command.nested_calls_allowed true false
dom.ipc.cpows.forbid-unsafe-from-browser true undefined
dom.ipc.cpows.log.enabled false undefined
dom.ipc.cpows.log.stack false undefined
dom.ipc.processPrelaunch.fission.number undefined 3
dom.link.disabled_attribute.enabled true undefined
dom.quotaManager.storageName undefined storage
dom.sidebar.enabled true false
dom.storage.next_gen false true
dom.targetBlankNoOpener.enabled false true
dom.window.content.untrusted.enabled true false
extensions.abuseReport.openDialog true undefined
extensions.webcompat-reporter.enabled false true
geo.provider-country.network.scan false undefined
geo.provider-country.network.url https://location.services.mozilla.com/v1/country?key=%MOZILLA_API_KEY% undefined
geo.provider.network.url https://www.googleapis.com/geolocation/v1/geolocate?key=%GOOGLE_LOCATION_SERVICE_API_KEY% https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%
gfx.webrender.quality.force-disable-sacrificing-subpixel-aa false undefined
gfx.webrender.quality.force-subpixel-aa-where-possible undefined false
identity.fxaccounts.useSessionTokensForOAuth undefined true
image.avif.use-dav1d undefined true
javascript.options.shared_memory false true
javascript.options.source_pragmas undefined true
javascript.options.wasm_multi_value undefined true
layout.animation.prerender.viewport-ratio-limit undefined 1.125
layout.animation.prerender.viewport-ratio-limit-x 1.125 undefined
layout.animation.prerender.viewport-ratio-limit-y 1.125 undefined
layout.css.aspect-ratio-number.enabled false undefined
layout.css.aspect-ratio.enabled undefined false
layout.css.file-chooser-button.enabled undefined false
layout.css.font-visibility.level undefined 3
layout.css.is-where-selectors.enabled false true
layout.css.moz-document.url-prefix-hack.enabled true undefined
layout.css.moz-focus-outer.enabled undefined true
layout.dynamic-reflow-roots.enabled false true
layout.dynamic-toolbar-max-height undefined 0
media.autoplay.blocking_policy undefined 0
media.autoplay.enabled.user-gestures-needed true undefined
media.cubeb.output_voice_routing undefined true
media.getusermedia.aecm_output_routing undefined 3
media.getusermedia.experimental_input_processing undefined false
media.navigator.audio.full_duplex true undefined
media.navigator.video.use_transport_cc false true
media.peerconnection.video.use_rtx undefined false
network.dns.disabled undefined false
network.http.sanitize-headers-in-logs undefined true
network.preload false true
network.preload-experimental false undefined
network.send_ODA_to_content_directly undefined true
pdfjs.handleOctetStream undefined true
permissions.isolateBy.privateBrowsing false true
permissions.postPrompt.animate true undefined
privacy.annotate_channels.strict_list.enabled false true
privacy.dynamic_firstparty.use_site undefined true
privacy.firstparty.isolate.use_site undefined false
privacy.partition.network_state undefined false
privacy.purge_trackers.enabled false true
privacy.purge_trackers.logging.level Error All
privacy.rejectForeign.allowList undefined
privacy.resistFingerprinting.randomDataOnCanvasExtract undefined true
security.allow_unsafe_parent_loads undefined false
security.block_Worker_with_wrong_mime false true
security.cancel_non_local_loads_triggered_by_systemprincipal undefined false
security.disallow_non_local_systemprincipal_in_tests undefined false
security.osreauthenticator.blank_password undefined false
security.osreauthenticator.password_last_changed_hi undefined 0
security.osreauthenticator.password_last_changed_lo undefined 0
security.remote_settings.crlite_filters.enabled false true
security.tls.version.min 1 3
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
toolkit.asyncshutdown.report_writes_after 20000 40000
toolkit.cosmeticAnimations.enabled true undefined
toolkit.telemetry.enabled false true
webextensions.storage.sync.kinto undefined true
widget.use-xdg-desktop-portal undefined false