This is difference between the default preference of Firefox 84.0 beta 3 and one of 83.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 83.0Firefox 84.0 beta 3
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/%LOCALE%/firefox/ https://www.mozilla.org/%LOCALE%/firefox/beta
apz.android.chrome_fling_physics.enabled true undefined
apz.fling_accel_interval_ms 500 undefined
apz.fling_accel_max_pause_interval_ms undefined 50
apz.fling_accel_min_fling_velocity undefined 1.5
apz.fling_accel_min_pan_velocity undefined 0.8
apz.fling_accel_min_velocity 1.5 undefined
apz.velocity_relevance_time_ms 150 100
browser.bookmarks.defaultLocation undefined toolbar
browser.cache.offline.storage.enable true false
browser.contentblocking.report.lockwise.mobile-android.url https://play.google.com/store/apps/details?id=mozilla.lockbox&referrer=utm_source%3Dprotection_report%26utm_content%3Dmobile_promotion undefined
browser.contentblocking.report.lockwise.mobile-ios.url https://apps.apple.com/app/id1314000270 undefined
browser.contentblocking.state-partitioning.mvp.ui.enabled undefined false
browser.defaultbrowser.notificationbar false true
browser.migrate.showBookmarksToolbarAfterMigration false true
browser.newtabpage.activity-stream.feeds.asrouterfeed true undefined
browser.newtabpage.activity-stream.improvesearch.topSiteSearchShortcuts.searchEngines google,amazon
browser.newtabpage.activity-stream.logowordmark.alwaysVisible undefined false
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.partnerlink.useAttributionURL false undefined
browser.startup.homepage.abouthome_cache.loglevel undefined Warn
browser.toolbars.bookmarks.2h2020 false true
browser.toolbars.bookmarks.visibility undefined newtab
devtools.contenttoolbox.fission true undefined
devtools.performance.recording.child.timeout_s undefined 0
doh-rollout.clearModeOnShutdown true false
dom.event.default_to_passive_wheel_listeners false true
dom.event.wheel-deltaMode-lines-to-pixel-scale undefined 30
dom.event.wheel-deltaMode-lines.always-disabled undefined false
dom.event.wheel-deltaMode-lines.disabled undefined false
dom.expose-incomplete-stylesheets undefined false
dom.image-lazy-loading.root-margin.bottom 0.0 300.0
dom.image-lazy-loading.root-margin.left 0.0 300.0
dom.image-lazy-loading.root-margin.right 0.0 300.0
dom.image-lazy-loading.root-margin.top 0.0 300.0
dom.innerSize.rounded undefined true
dom.input_event.allow_to_cancel_set_user_input undefined false
dom.input_events.beforeinput.enabled false true
dom.inputevent.data.enabled true undefined
dom.inputevent.datatransfer.enabled true undefined
dom.inputevent.inputtype.enabled true undefined
dom.storage.next_gen false true
dom.window.content.untrusted.enabled true false
extensions.pocket.loggedOutVariant undefined control
extensions.webcompat-reporter.enabled false true
formhelper.autozoom.force-disable.test-only undefined false
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.e10s.font-list.shared false true
gfx.webrender.debug.compact-profiler false undefined
gfx.webrender.debug.new-frame-indicator false undefined
gfx.webrender.debug.new-scene-indicator false undefined
gfx.webrender.debug.profiler-ui undefined Default
gfx.webrender.debug.smart-profiler false undefined
gfx.webrender.software undefined false
gfx.webrender.software.d3d11 undefined true
javascript.options.wasm_ionjit true undefined
javascript.options.wasm_optimizingjit undefined true
layout.accessiblecaret.transition-duration undefined 250.0
layout.css.is-and-where-better-error-recovery.enabled false true
layout.display-list.improve-fragmentation undefined false
layout.display-list.retain.verify false undefined
layout.display-list.retain.verify.order false undefined
layout.dynamic-reflow-roots.enabled false true
mathml.mfenced_element.disabled true undefined
media.ffmpeg.dmabuf-textures.disabled undefined false
media.ffmpeg.dmabuf-textures.enabled false undefined
media.peerconnection.dtls.version.min 770 771
media.peerconnection.mtransport_process false true
media.rdd-ffmpeg.enabled undefined false
media.rdd-ffvpx.enabled undefined false
media.rdd-theora.enabled undefined false
media.rdd-vpx.enabled undefined false
media.sanity-test.disabled undefined false
media.videocontrols.picture-in-picture.allow-multiple undefined false
media.videocontrols.picture-in-picture.video-toggle.mode 2 undefined
network.connectivity-service.nat64-prefix undefined
network.dns.use_https_rr_for_speculative_connection undefined false
network.preload false true
network.process.enabled false true
permissions.isolateBy.privateBrowsing false true
print.cups.monochrome.extra_settings undefined
print.tab_modal.enabled false true
privacy.annotate_channels.strict_list.enabled false true
privacy.antitracking.testing undefined false
privacy.partition.network_state false true
security.bad_cert_domain_error.url_fix_enabled false true
security.disallow_non_local_systemprincipal_in_tests undefined false
security.external_protocol_requires_permission undefined true
security.intermediate_preloading_healer.enabled false true
security.remote_settings.crlite_filters.enabled false true
security.remote_settings.intermediates.downloads_per_poll 100 5000
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
signon.management.page.hideMobileFooter false undefined
signon.management.page.mobileAndroidURL https://app.adjust.com/6tteyjo?redirect=https%3A%2F%2Fplay.google.com%2Fstore%2Fapps%2Fdetails%3Fid%3Dmozilla.lockbox&utm_campaign=Desktop&utm_adgroup=InProduct&utm_creative= undefined
signon.management.page.mobileAppleURL https://app.adjust.com/6tteyjo?redirect=https%3A%2F%2Fitunes.apple.com%2Fapp%2Fid1314000270%3Fmt%3D8&utm_campaign=Desktop&utm_adgroup=InProduct&utm_creative= undefined
toolkit.telemetry.enabled false true