This is difference between the default preference of Firefox 87.0 beta 4 and one of 86.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 86.0Firefox 87.0 beta 4
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.nonwr.activate_all_scroll_frames undefined false
apz.nonwr.activate_all_scroll_frames_when_fission undefined false
apz.prefer_jank_minimal_displayports undefined true
apz.wr.activate_all_scroll_frames undefined false
apz.wr.activate_all_scroll_frames_when_fission undefined true
browser.aboutwelcome.skipFocus undefined false
browser.cache.cache_isolation false undefined
browser.defaultbrowser.notificationbar false true
browser.library.activity-stream.enabled true undefined
browser.newtabpage.activity-stream.improvesearch.topSiteSearchShortcuts.searchEngines google,amazon
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.preferences.exposeHTTPSOnly true undefined
browser.proton.toolbar.version undefined 0
browser.quitShortcut.disabled undefined false
browser.topsites.useRemoteSetting false true
browser.urlbar.imeCompositionClosesPanel true undefined
browser.urlbar.keepPanelOpenDuringImeComposition undefined false
browser.urlbar.quicksuggest.enabled undefined false
browser.urlbar.quicksuggest.helpURL undefined https://www.mozilla.org/
browser.urlbar.quicksuggest.onboardingCount undefined 0
browser.urlbar.quicksuggest.onboardingMaxCount undefined 5
browser.urlbar.showSearchSuggestionsFirst undefined true
browser.urlbar.suggest.quicksuggest undefined true
devtools.inspector.color-scheme-simulation.enabled false true
devtools.inspector.new-rulesview.enabled false undefined
devtools.target-switching.enabled true undefined
dom.enable_event_timing undefined false
dom.events.asyncClipboard.clipboardItem undefined false
dom.events.asyncClipboard.dataTransfer false undefined
dom.expose-incomplete-stylesheets false undefined
dom.formdata.event.enabled true undefined
dom.forms.requestsubmit.enabled true undefined
dom.forms.submit.trusted_event_only undefined false
dom.input_events.beforeinput.enabled false true
dom.quotaManager.directoryRemovalOrRenaming.delayMs undefined 200
dom.quotaManager.directoryRemovalOrRenaming.maxRetries undefined 0
dom.storage.next_gen false true
dom.w3c_pointer_events.enabled true undefined
dom.w3c_pointer_events.multiprocess.android.enabled true undefined
dom.window.content.untrusted.enabled true false
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.force_srgb undefined false
gfx.e10s.font-list.shared false true
gfx.prefer-x11-egl undefined false
gfx.webrender.debug.batched-texture-uploads undefined false
gfx.webrender.debug.draw-calls-for-texture-copy undefined false
gfx.webrender.fallback.basic undefined true
gfx.webrender.fallback.software undefined false
gfx.webrender.prefer-robustness undefined true
image.avif.enabled false true
javascript.options.large_arraybuffers undefined false
javascript.options.off_thread_parse_global false undefined
javascript.options.wasm_simd_wormhole undefined false
layers.advanced.basic-layer.enabled false undefined
layers.advanced.fission.enabled false undefined
layers.mlgpu.enable-buffer-cache true undefined
layers.mlgpu.enable-buffer-sharing true undefined
layers.mlgpu.enable-clear-view true undefined
layers.mlgpu.enable-cpu-occlusion true undefined
layers.mlgpu.enable-depth-buffer false undefined
layers.mlgpu.enable-invalidation true undefined
layers.mlgpu.enable-on-windows7 false undefined
layers.mlgpu.enabled false undefined
layout.css.caption-side-non-standard.enabled undefined false
layout.css.focus-visible.enabled true undefined
layout.css.outline-follows-border-radius.enabled undefined false
layout.dynamic-reflow-roots.enabled false true
media.peerconnection.mtransport_process false true
media.videocontrols.keyboard-tab-to-all-controls false true
network.cache.frecency_array_check_enabled false true
network.cors_preflight.allow_client_cert undefined false
network.dns.limit_253_chars undefined true
network.http.referer.defaultPolicy 3 2
network.process.enabled false true
network.proxy.reload_pac_delay undefined 2000
network.tcp.tcp_fastopen_consecutive_failure_limit 5 undefined
network.tcp.tcp_fastopen_enable false undefined
network.tcp.tcp_fastopen_http_check_for_stalls_only_if_idle_for 10 undefined
network.tcp.tcp_fastopen_http_stalls_limit 3 undefined
network.tcp.tcp_fastopen_http_stalls_timeout 20 undefined
network.trr.confirmation_telemetry_enabled undefined true
network.trr.confirmation_timeout_ms undefined 6000
network.trr.early-AAAA false undefined
network.trr.odoh.enabled undefined false
network.trr.odoh.proxy_uri undefined
network.trr.odoh.target_host undefined
network.trr.odoh.target_path undefined
network.trr.wait-for-A-and-AAAA true undefined
network.trr.wait-for-confirmation undefined false
permissions.isolateBy.privateBrowsing false true
print.more-settings.open undefined false
print.tab_modal.enabled false true
privacy.annotate_channels.strict_list.enabled false true
prompts.contentPromptSubDialog false true
prompts.windowPromptSubDialog undefined false
security.allow_parent_unrestricted_js_loads true false
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
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
services.sync.prefs.sync.browser.urlbar.matchBuckets true undefined
services.sync.prefs.sync.browser.urlbar.resultBuckets undefined true
services.sync.prefs.sync.browser.urlbar.showSearchSuggestionsFirst undefined true
telemetry.fog.test.activity_limit undefined 120
telemetry.fog.test.inactivity_limit undefined 1200
timer.ignore_sleep_wake_notifications undefined false
toolkit.telemetry.enabled false true
ui.use_native_colors true undefined
webgl.oop.via-pcq false undefined
widget.gtk.non-native.round-thumb undefined true
widget.gtk.non-native.scrollbar.allow-buttons undefined false
widget.gtk.non-native.scrollbar.normal-size undefined 12
widget.gtk.non-native.scrollbar.thin-size undefined 6
widget.gtk.non-native.scrollbar.thumb-cross-size undefined 40
widget.gtk.non-native.scrollbar.thumb-size undefined 0.75
widget.non-native.use-theme-accent undefined true
widget.wayland.opaque-region.enabled undefined true
widget.wayland.use-opaque-region false undefined