This is difference between the default preference of Firefox 79.0 beta 6 and one of 78.0.2. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 78.0.2Firefox 79.0 beta 6
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.notifyDuringDownload undefined false
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/%LOCALE%/firefox/ https://www.mozilla.org/%LOCALE%/firefox/beta
apz.mvm.force-enabled undefined false
apz.windows.force_disable_direct_manipulation true false
browser.cache.offline.storage.enable true false
browser.download.alwaysOpenInSystemViewerContextMenuItem undefined true
browser.download.openInSystemViewerContextMenuItem undefined true
browser.find.anonymous_content.enabled true undefined
browser.fixup.dns_first_for_single_words undefined false
browser.navigation.requireUserInteraction undefined false
browser.newtabpage.activity-stream.asrouter.providers.message-groups {"id":"message-groups","enabled":false,"type":"remote-settings","bucket":"message-groups","updateCycleInMs":3600000} {"id":"message-groups","enabled":true,"type":"remote-settings","bucket":"message-groups","updateCycleInMs":3600000}
browser.newtabpage.activity-stream.telemetry.structuredIngestion true undefined
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.preferences.experimental undefined false
browser.region.update.enabled undefined true
browser.search.modernConfig false true
browser.tabs.remote.useCrossOriginEmbedderPolicy false true
browser.tabs.remote.useCrossOriginOpenerPolicy false true
browser.urlbar.delay 50 undefined
browser.urlbar.formatting.enabled true undefined
browser.urlbar.restyleSearches false undefined
browser.urlbar.richSuggestions.tail false true
browser.urlbar.update1.interventions true undefined
browser.urlbar.update1.searchTips true undefined
browser.urlbar.usepreloadedtopurls.enabled false undefined
browser.urlbar.usepreloadedtopurls.expire_days 14 undefined
devtools.accessibility.auto-init.enabled false true
devtools.application.enabled false true
devtools.contenttoolbox.fission false true
devtools.debugger.features.async-captured-stacks false true
devtools.debugger.features.async-live-stacks true false
devtools.debugger.features.windowless-service-workers false true
devtools.netmonitor.msg.displayed-messages.limit undefined 500
devtools.netmonitor.msg.messageDataLimit undefined 100000
devtools.netmonitor.msg.payload-preview-height undefined 128
devtools.netmonitor.msg.visibleColumns undefined ["data", "time"]
devtools.netmonitor.ws.displayed-frames.limit 500 undefined
devtools.netmonitor.ws.messageDataLimit 100000 undefined
devtools.netmonitor.ws.payload-preview-height 128 undefined
devtools.netmonitor.ws.visibleColumns ["data", "time"] undefined
devtools.overflow.debugging.enabled undefined false
devtools.responsive.browserUI.enabled false true
doh-rollout.provider-steering.enabled undefined false
doh-rollout.provider-steering.provider-list undefined [{ "name": "comcast", "canonicalName": "doh-discovery.xfinity.com", "uri": "https://doh.xfinity.com/dns-query" }]
dom.arena_allocator.enabled false true
dom.document.exec_command.nested_calls_allowed true false
dom.forms.enterkeyhint undefined false
dom.ipc.processPrelaunch.fission.number undefined 3
dom.ipc.tabs.shutdownTimeoutSecs 5 20
dom.largeAllocation.testing.allHttpLoads false undefined
dom.manifest.enabled false true
dom.postMessage.sharedArrayBuffer.withCOOP_COEP false true
dom.script_loader.external_scripts.speculate_async.enabled undefined false
dom.script_loader.external_scripts.speculate_link_preload.enabled undefined false
dom.script_loader.external_scripts.speculate_non_parser_inserted.enabled undefined false
dom.script_loader.external_scripts.speculative_omt_parse.enabled undefined false
dom.storage.next_gen false true
dom.targetBlankNoOpener.enabled false true
dom.window.content.untrusted.enabled true false
editor.white_space_normalization.blink_compatible undefined false
extensions.webcompat-reporter.enabled false true
fission.rebuild_frameloaders_on_remoteness_change true undefined
geo.provider.network.compare.url undefined https://www.googleapis.com/geolocation/v1/geolocate?key=%GOOGLE_LOCATION_SERVICE_API_KEY%
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.precache-shaders undefined false
image.http.accept image/webp,*/*
intl.charset.detector chrome://global/locale/intl.properties undefined
intl.charset.detector.iso2022jp.allowed true undefined
intl.charset.detector.ng.enabled true undefined
intl.charset.fallback.override undefined
intl.charset.fallback.tld true undefined
javascript.options.asyncstack false true
javascript.options.asyncstack_capture_debuggee_only undefined true
javascript.options.experimental.weakrefs.expose_cleanupSome undefined false
javascript.options.wasm_gc undefined false
javascript.options.weakrefs undefined true
jsloader.shareGlobal true undefined
layout.css.prefers-color-scheme-no-preference.enabled undefined false
layout.css.scrollbar-color.enabled true undefined
layout.css.scrollbar-width.enabled true undefined
layout.dynamic-reflow-roots.enabled false true
mathml.stixgeneral_operator_stretching.disabled undefined false
media.clockdrift.buffering undefined 5
media.navigator.video.use_transport_cc false true
media.peerconnection.ice.obfuscate_host_addresses.blocklist undefined
media.peerconnection.ice.obfuscate_host_addresses.whitelist undefined
media.peerconnection.video.use_rtx false true
media.peerconnection.video.use_rtx.blocklist undefined *.google.com
network.cookie.sameSite.schemeful undefined false
network.preload false true
network.send_ODA_to_content_directly true false
permissions.isolateBy.privateBrowsing false true
plugin.mousewheel.enabled undefined true
print.always_print_silent undefined false
privacy.annotate_channels.strict_list.enabled false true
privacy.partition.network_state.connection_with_proxy undefined false
privacy.purge_trackers.consider_entity_list undefined false
privacy.purge_trackers.enabled false true
privacy.purge_trackers.logging.level Error Warn
privacy.userInteraction.expiration 2592000 3888000
security.aboutcertificate.enabled true undefined
security.allow_disjointed_external_uri_loads true false
security.block_Worker_with_wrong_mime false true
security.disallow_non_local_systemprincipal_in_tests undefined false
security.intermediate_preloading_healer.enabled undefined true
security.intermediate_preloading_healer.timer_interval_ms undefined 300000
security.remote_settings.crlite_filters.enabled false true
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
signon.backup.enabled undefined false
signon.capture.inputChanges.enabled undefined true
telemetry.fog.test.localhost_port undefined 0
toolkit.asyncshutdown.report_writes_after 40000 undefined
toolkit.shutdown.fastShutdownStage 0 1
toolkit.telemetry.enabled false true
toolkit.telemetry.isGeckoViewMode false undefined
ui.mouse.radius.reposition undefined false
webextensions.storage.sync.kinto true false
widget.wayland-dmabuf-video-textures.enabled undefined false
widget.wayland-dmabuf-webgl.enabled false true