This is difference between the default preference of Firefox 91.0 beta 3 and one of 90.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 90.0Firefox 91.0 beta 3
app.normandy.test-prefs.bool undefined false
app.normandy.test-prefs.integer undefined 0
app.normandy.test-prefs.string undefined
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.releaseNotesURL.aboutDialog https://www.mozilla.org/%LOCALE%/firefox/%VERSION%/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-desktop&utm_campaign=about-dialog https://www.mozilla.org/%LOCALE%/firefox/%VERSION%beta/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-desktop&utm_campaign=about-dialog
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.doubletapzoom.defaultzoomin undefined 1.2
browser.cache.offline.enable true false
browser.display.windows.non_native_menus undefined 2
browser.messaging-system.personalized-cfr.score-threshold 5000 undefined
browser.messaging-system.personalized-cfr.scores {} undefined
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.places.interactions.typing_timeout_ms undefined 3000
browser.proton.contextmenus.enabled true undefined
browser.proton.doorhangers.enabled true undefined
browser.proton.modals.enabled true undefined
browser.tabs.remote.useOriginAttributesInRemoteType true undefined
doh-rollout.provider-steering.enabled true undefined
doh-rollout.provider-steering.provider-list [{ "name": "comcast", "canonicalName": "doh-discovery.xfinity.com", "uri": "https://doh.xfinity.com/dns-query" }] undefined
doh-rollout.trr-selection.enabled false undefined
dom.events.coalesce.touchmove undefined false
dom.image-lazy-loading.root-margin.bottom 300.0 1000.0
dom.image-lazy-loading.root-margin.left 300.0 1000.0
dom.image-lazy-loading.root-margin.right 300.0 1000.0
dom.image-lazy-loading.root-margin.top 300.0 1000.0
dom.input_events.strict_input_vsync_alignment false true
dom.ipc.avoid-gtk undefined true
dom.ipc.processPrelaunch.lowmem_mb undefined 4096
dom.script_loader.full_parse undefined false
dom.security.https_first_pbm false true
dom.security.https_only_fire_http_request_background_timer_ms undefined 3000
dom.select_events.enabled true undefined
dom.serviceWorkers.navigationPreload.enabled undefined false
dom.serviceWorkers.parent_intercept true undefined
dom.storage.next_gen false true
dom.visualviewport.enabled false true
dom.w3c_pointer_events.implicit_capture false true
dom.window.clientinformation.enabled undefined true
dom.window.content.untrusted.enabled true false
dom.window.sidebar.enabled undefined false
extensions.allowPrivateBrowsingByDefault false undefined
extensions.blocklist.useMLBF true undefined
extensions.blocklist.useMLBF.stashes true undefined
extensions.webcompat-reporter.enabled false true
extensions.webidl-api.expose_mock_interface undefined false
fission.bfcacheInParent false true
general.smoothScroll.mouseWheel.migrationPercent 100 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.fallback.basic true false
gfx.webrender.low-quality-pinch-zoom undefined false
gfx.webrender.software.unaccelerated-widget.allow false true
image.avif.apply_transforms undefined true
intl.charset.detector.ng.in.enabled true undefined
intl.charset.detector.ng.jp.enabled true undefined
intl.charset.detector.ng.lk.enabled true undefined
javascript.options.external_thread_pool undefined true
javascript.options.site_based_pretenuring undefined true
javascript.options.wasm_simd_wormhole undefined false
layout.css.d-property.enabled undefined false
layout.css.element-content-none.enabled undefined false
layout.css.fit-content-function.enabled undefined false
layout.css.font-size-adjust.basis.enabled undefined false
layout.css.moz-outline-radius.enabled false undefined
layout.css.outline-follows-border-radius.enabled true undefined
layout.dynamic-reflow-roots.enabled false true
layout.extra-tick.minimum-ms -1 4
marionette.prefs.recommended true undefined
media.peerconnection.mtransport_process false true
mousewheel.system_scroll_override.enabled false true
network.cache.frecency_array_check_enabled false true
network.dns.copy_string_before_call undefined true
network.dns.force_waiting_https_rr undefined false
network.dns.upgrade_with_https_rr false true
network.dns.use_https_rr_as_altsvc false true
network.http.windows-sso.enabled undefined false
network.jar.record_failure_reason false true
network.process.enabled false true
network.proxy.default_pac_script_socks_version undefined 4
network.trr.default_provider_uri undefined https://mozilla.cloudflare-dns.com/dns-query
network.trr.resolvers [{ "name": "Cloudflare", "url": "https://mozilla.cloudflare-dns.com/dns-query" },{ "name": "NextDNS", "url": "https://firefox.dns.nextdns.io/" }] undefined
network.trr.skip-check-for-blocked-host undefined false
network.trr.uri https://mozilla.cloudflare-dns.com/dns-query
places.forgetThisSite.clearByBaseDomain false true
privacy.annotate_channels.strict_list.enabled false true
remote.active-protocols undefined 2
remote.prefs.recommended undefined true
screenshots.browser.component.enabled undefined false
security.caps.allow_uri_is_ui_resource_in_checkloaduriflags false undefined
security.cert_pinning.hpkp.enabled false undefined
security.cert_pinning.max_max_age_seconds 5184000 undefined
security.cert_pinning.process_headers_from_non_builtin_roots false undefined
security.disallow_non_local_systemprincipal_in_tests undefined false
security.intermediate_preloading_healer.enabled false true
security.remote_settings.crlite_filters.enabled false true
services.blocklist.addons-mlbf.collection addons-bloomfilters undefined
services.blocklist.addons-mlbf.signer remote-settings.content-signature.mozilla.org undefined
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
services.sync.prefs.sync.browser.urlbar.resultBuckets true undefined
signon.improvedPasswordRules.enabled undefined true
signon.usernameOnlyForm.enabled undefined true
toolkit.telemetry.enabled false true
widget.gtk.alt-theme.dark false true
widget.gtk.follow-firefox-theme undefined true
widget.system-colors-follow-theme false undefined
widget.wayland.multi-buffer-software-backend.enabled undefined false