This is difference between the default preference of Firefox 77.0 beta 6 and one of 76.0.1. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 76.0.1Firefox 77.0 beta 6
accessibility.ARIAReflection.enabled undefined false
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/firefox/beta
apz.frame_delay.enabled true undefined
apz.windows.force_disable_direct_manipulation undefined true
browser.aboutwelcome.log warn undefined
browser.cache.offline.storage.enable true false
browser.contentblocking.report.monitor.how_it_works.url https://support.mozilla.org/1/firefox/%VERSION%/%OS%/%LOCALE%/monitor-faq https://monitor.firefox.com/about
browser.find.anonymous_content.enabled undefined true
browser.fixup.dns_first_for_single_words false undefined
browser.fixup.typo.scheme true undefined
browser.newtabpage.activity-stream.asrouter.providers.message-groups {"id":"message-groups","enabled":true,"type":"remote-settings","bucket":"message-groups","updateCycleInMs":3600000} {"id":"message-groups","enabled":false,"type":"remote-settings","bucket":"message-groups","updateCycleInMs":3600000}
browser.newtabpage.activity-stream.asrouter.providers.messaging-experiments undefined {"id":"messaging-experiments","enabled":true,"type":"remote-experiments","messageGroups":["cfr","whats-new-panel","moments-page","snippets","cfr-fxa"],"updateCycleInMs":3600000}
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.osKeyStore.loglevel Warn undefined
browser.safebrowsing.provider.google.reportURL https://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&site= https://safebrowsing.google.com/safebrowsing/diagnostic?site=
browser.safebrowsing.provider.google4.reportURL https://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&site= https://safebrowsing.google.com/safebrowsing/diagnostic?site=
browser.stopReloadAnimation.enabled true undefined
browser.tabs.documentchannel.parent-initiated undefined true
browser.tabs.documentchannel.ppdc undefined true
browser.tabs.multiselect true undefined
browser.tabs.remote.allowLinkedWebInFileUriProcess false undefined
browser.tabs.remote.separatePrivilegedContentProcess false true
browser.tabs.showAudioPlayingIcon true undefined
browser.ui.scroll-toolbar-threshold 10 undefined
browser.urlbar.maxCharsForSearchSuggestions 20 100
browser.urlbar.oneOffSearches true undefined
browser.urlbar.restyleSearches undefined false
browser.urlbar.update1 true undefined
browser.urlbar.update1.view.stripHttps true undefined
browser.xul.error_pages.enabled true undefined
devtools.contenttoolbox.webconsole.input.context undefined false
devtools.debugger.features.frame-step undefined true
devtools.debugger.source-maps-enabled true undefined
devtools.experiment.f12.shortcut_disabled undefined false
devtools.inspector.compatibility.target-browsers undefined
devtools.inspector.use-new-box-model-highlighter false undefined
devtools.netmonitor.columnsData [{"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":"url","minWidth":30,"width":25}, {"name":"cause","minWidth":30,"width":10},{"name":"initiator","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":15}] [{"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":"url","minWidth":30,"width":25},{"name":"initiator","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":15}]
devtools.netmonitor.visibleColumns ["status","method","domain","file","cause","type","transferred","contentSize","waterfall"] ["status","method","domain","file","initiator","type","transferred","contentSize","waterfall"]
dom.arena_allocator.enabled false true
dom.document.exec_command.nested_calls_allowed true false
dom.mozBrowserFramesEnabled true undefined
dom.registerProtocolHandler.insecure.enabled false undefined
dom.security.https_only_mode.upgrade_local undefined false
dom.security.https_only_mode.upgrade_onion undefined false
dom.security.https_only_mode_ever_enabled undefined false
dom.storage.next_gen false true
dom.targetBlankNoOpener.enabled false true
dom.window.content.untrusted.enabled undefined false
editor.truncate_user_pastes undefined false
extensions.blocklist.addonItemURL undefined https://addons.mozilla.org/%LOCALE%/%APP%/blocked-addon/%addonID%/%addonVersion%/
extensions.blocklist.useMLBF undefined false
extensions.blocklist.useMLBF.stashes undefined false
extensions.webcompat-reporter.enabled false true
extensions.webservice.discoverURL https://discovery.addons.mozilla.org/%LOCALE%/firefox/discovery/pane/%VERSION%/%OS%/%COMPATIBILITY_MODE% 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.color_management.mode 2 1
gfx.omta.background-color false true
gfx.vsync.force-disable-waitforvblank undefined false
gfx.vsync.use-waitforvblank false undefined
gfx.webrender.enable-item-cache false true
gfx.webrender.gl-debug-message-critical-note undefined false
gfx.webrender.gl-debug-message-print undefined false
gfx.webrender.use-optimized-shaders undefined true
identity.sync.useOAuthForSyncToken undefined false
image.avif.enabled undefined false
image.honor-orientation-metadata undefined true
image.honor_orientation_metadata.natural_size undefined true
javascript.options.mem.gc_avoid_interrupt_factor 100 undefined
javascript.options.mem.gc_dynamic_heap_growth true undefined
javascript.options.mem.gc_dynamic_mark_slice true undefined
javascript.options.mem.gc_high_frequency_heap_growth_max 300 undefined
javascript.options.mem.gc_high_frequency_heap_growth_min 150 undefined
javascript.options.mem.gc_high_frequency_high_limit_mb 500 undefined
javascript.options.mem.gc_high_frequency_large_heap_growth undefined 150
javascript.options.mem.gc_high_frequency_low_limit_mb 100 undefined
javascript.options.mem.gc_high_frequency_small_heap_growth undefined 300
javascript.options.mem.gc_large_heap_incremental_limit undefined 110
javascript.options.mem.gc_large_heap_size_min_mb undefined 500
javascript.options.mem.gc_non_incremental_factor 112 undefined
javascript.options.mem.gc_small_heap_incremental_limit undefined 140
javascript.options.mem.gc_small_heap_size_max_mb undefined 100
javascript.options.shared_memory false true
javascript.options.wasm_reftypes undefined true
layout.css.always-repaint-on-unvisited false true
layout.css.contain.enabled true undefined
layout.css.grid-template-masonry-value.enabled undefined false
layout.css.image-orientation.initial-from-image false true
layout.css.is-where-selectors.enabled undefined false
layout.css.moz-document.url-prefix-hack.enabled true false
layout.css.notify-of-unvisited false true
layout.dynamic-reflow-roots.enabled false true
media.getdisplaymedia.enabled undefined true
media.navigator.video.use_transport_cc false true
media.testing-only-events undefined false
network.auth.confirmAuth.enabled false true
network.data.max-uri-length-mobile undefined 2097152
network.disable.ipc.security true undefined
network.http.http3.default-qpack-table-size 65536 0
pdfjs.enablePermissions undefined false
permissions.isolateBy.privateBrowsing false true
privacy.annotate_channels.strict_list.enabled false true
privacy.purge_trackers.enabled false true
privacy.purge_trackers.logging.enabled false undefined
privacy.purge_trackers.logging.level Warn All
privacy.restrict3rdpartystorage.expiration_visited undefined 2592000
privacy.restrict3rdpartystorage.heuristic.recently_visited undefined true
privacy.restrict3rdpartystorage.heuristic.recently_visited_time undefined 600
privacy.trackingprotection.testing.report_blocked_node undefined false
prompts.defaultModalType undefined 3
prompts.modalType.confirmAuth undefined 2
prompts.modalType.insecureFormSubmit undefined 2
security.block_Worker_with_wrong_mime false true
security.disallow_non_local_systemprincipal_in_tests undefined false
security.identityblock.show_extended_validation false undefined
security.remote_settings.crlite_filters.enabled false true
security.tls.version.min 1 3
services.blocklist.addons-mlbf.checked undefined 0
services.blocklist.addons-mlbf.collection undefined addons-bloomfilters
services.blocklist.addons-mlbf.signer undefined remote-settings.content-signature.mozilla.org
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
services.sync.prefs.sync.intl.regional_prefs.use_os_locales undefined true
signon.management.overrideURI about:logins?filter=%DOMAIN% undefined
signon.management.page.os-auth.enabled true false
signon.showAutoCompleteImport undefined
toolkit.asyncshutdown.report_writes_after undefined 20000
toolkit.osKeyStore.loglevel undefined Warn
toolkit.telemetry.enabled false true
webgl.prototype.ipc-pcq undefined 0