This is difference between the default preference of Firefox 89.0 beta 6 and one of 88.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 88.0Firefox 89.0 beta 6
app.feedback.baseURL https://input.mozilla.org/%LOCALE%/feedback/%APP%/%VERSION%/ https://ideas.mozilla.org/
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.gtk.touchpad_pinch.three_fingers.enabled undefined false
apz.mac.enable_double_tap_zoom_touchpad_gesture false true
apz.overscroll.damping undefined 1.1
apz.overscroll.max_velocity undefined 10.0
apz.overscroll.spring_stiffness undefined 200.0
apz.overscroll.test_async_scroll_offset.enabled undefined false
apz.windows.check_for_pan_gesture_conversion undefined true
browser.aboutwelcome.design undefined
browser.aboutwelcome.skipFocus false true
browser.cache.offline.enable true false
browser.compactmode.show undefined false
browser.contentblocking.features.strict tp,tpPrivate,cookieBehavior5,cm,fp,stp,lvl2 tp,tpPrivate,cookieBehavior5,cookieBehaviorPBM5,cm,fp,stp,lvl2
browser.contentblocking.report.vpn_platforms win win,mac,linux
browser.contentblocking.report.vpn_regions us,ca,nz,sg,my,gb us,ca,nz,sg,my,gb,de,fr
browser.ctrlTab.recentlyUsedOrder true undefined
browser.ctrlTab.sortByRecentlyUsed undefined false
browser.defaultbrowser.notificationbar false undefined
browser.defaultbrowser.notificationbar.checkcount 0 undefined
browser.defaultbrowser.notificationbar.checklimit 10000 undefined
browser.menu.showViewImageInfo undefined false
browser.newtabpage.activity-stream.feeds.section.highlights true false
browser.newtabpage.activity-stream.improvesearch.handoffToAwesomebar false true
browser.newtabpage.activity-stream.improvesearch.topSiteSearchShortcuts.searchEngines google,amazon
browser.newtabpage.activity-stream.logowordmark.alwaysVisible false true
browser.newtabpage.activity-stream.newNewtabExperience.enabled false true
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.opaqueResponseBlocking undefined false
browser.proton.contextmenus.enabled undefined true
browser.proton.doorhangers.enabled undefined true
browser.proton.enabled false true
browser.proton.modals.enabled undefined true
browser.proton.places-tooltip.enabled undefined false
browser.startup.upgradeDialog.enabled undefined true
browser.tabs.extraDragSpace false undefined
browser.tabs.secondaryTextUnsupportedLocales undefined ar,bn,bo,ckb,fa,gu,he,hi,ja,km,kn,ko,lo,mr,my,ne,pa,si,ta,te,th,ur,zh
browser.theme.dark-toolbar-theme undefined false
browser.urlbar.suggest.calculator undefined false
captivedetect.canonicalContent success <meta http-equiv="refresh" content="0;url=https://support.mozilla.org/kb/captive-portal"/>
captivedetect.canonicalURL http://detectportal.firefox.com/success.txt http://detectportal.firefox.com/canonical.html
devtools.target-switching.server.enabled undefined false
doh-rollout.provider-steering.enabled false true
dom.enable_event_timing false true
dom.forms.datetime-local undefined false
dom.forms.datetime-local.widget undefined false
dom.image-lazy-loading.root-margin.bottom 300.0 600.0
dom.image-lazy-loading.root-margin.left 300.0 600.0
dom.image-lazy-loading.root-margin.right 300.0 600.0
dom.image-lazy-loading.root-margin.top 300.0 600.0
dom.input_events.strict_input_vsync_alignment undefined false
dom.security.enforceIPCBasedPrincipalVetting undefined true
dom.security.https_only_mode_https_first undefined false
dom.security.https_only_mode_https_first_pbm undefined false
dom.storage.next_gen false true
dom.w3c_pointer_events.implicit_capture false true
dom.window.content.untrusted.enabled true false
extensions.pocket.showHome undefined true
extensions.screenshots.upload-disabled false undefined
extensions.webcompat-reporter.enabled false true
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.color_management.native_srgb undefined false
gfx.e10s.font-list.shared false true
gfx.webrender.fallback.basic true false
gfx.webrender.fallback.software false true
gfx.webrender.program-binary-disk undefined false
gfx.webrender.software.unaccelerated-widget.allow false true
gfx.webrender.software.unaccelerated-widget.force false undefined
image.honor-orientation-metadata true undefined
image.honor_orientation_metadata.natural_size true undefined
image.mem.shared.unmap.force-enabled undefined false
image.mem.shared.unmap.min_expiration_ms undefined 60000
image.mem.shared.unmap.min_threshold_kb undefined 100
intl.l10n.pseudo undefined
javascript.options.experimental.top_level_await undefined true
javascript.options.inlining_bytecode_max_length undefined 130
javascript.options.large_arraybuffers false true
javascript.options.wasm_gc false undefined
javascript.options.wasm_multi_value true undefined
javascript.options.wasm_reftypes true undefined
javascript.options.wasm_simd false true
javascript.options.wasm_simd_wormhole undefined false
layout.css.aspect-ratio.enabled false true
layout.css.bloom-filter-attribute-names.enabled undefined true
layout.css.font-metrics-overrides.enabled undefined true
layout.css.forced-colors.enabled false true
layout.css.image-orientation.initial-from-image true undefined
layout.css.moz-outline-radius.enabled undefined false
layout.css.page-size.enabled undefined false
layout.css.prefers-color-scheme-no-preference.enabled false undefined
layout.css.size-adjust.enabled undefined false
layout.css.width-and-height-map-to-aspect-ratio.enabled true undefined
layout.dynamic-reflow-roots.enabled false true
layout.use-plugin-fallback undefined false
media.peerconnection.mtransport_process false true
network.cache.frecency_array_check_enabled false true
network.cookie.cookieBehavior.pbmode undefined 5
network.data.max-uri-length-mobile 2097152 undefined
network.http.http3.enabled false true
network.jar.record_failure_reason false true
network.process.enabled false true
network.trr.bootstrapAddress undefined
nglayout.initialpaint.delay undefined 5
nglayout.initialpaint.delay_in_oopif undefined 5
pdfjs.enablePrintAutoRotate false true
privacy.annotate_channels.strict_list.enabled false true
privacy.webrtc.deviceGracePeriodTimeoutMs 50000 3600000
prompts.contentPromptSubDialog false true
prompts.windowPromptSubDialog false true
security.disallow_non_local_systemprincipal_in_tests undefined false
security.intermediate_preloading_healer.enabled false true
security.osclientcerts.autoload false true
security.remote_settings.crlite_filters.enabled false true
security.tls.enable_delegated_credentials false true
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
services.sync.prefs.sync.browser.ctrlTab.recentlyUsedOrder true undefined
services.sync.prefs.sync.browser.ctrlTab.sortByRecentlyUsed undefined true
services.sync.prefs.sync.browser.menu.showViewImageInfo undefined true
signon.management.page.showPasswordSyncNotification true undefined
signon.recipes.remoteRecipes.enabled undefined true
signon.recipes.remoteRecipesEnabled true undefined
signon.relatedRealms.enabled undefined false
toolkit.telemetry.ecosystemtelemetry.enabled false undefined
toolkit.telemetry.enabled false true
ui.textHighlightBackground undefined #ef0fff
ui.textHighlightForeground undefined #ffffff
ui.textSelectBackgroundAttention undefined #38d878
ui.textSelectBackgroundDisabled undefined #b0b0b0
widget.content.allow-gtk-dark-theme.accent undefined true
widget.content.allow-gtk-dark-theme.scrollbar undefined false
widget.content.allow-gtk-dark-theme.scrollbar_active undefined true
widget.content.allow-gtk-dark-theme.selection undefined true
widget.non-native-theme.enabled false true
widget.non-native-theme.scrollbar.active-always-themed undefined true
widget.remote-look-and-feel true undefined
widget.wayland-smooth-rendering false 1