This is difference between the default preference of Firefox 88.0 beta 6 and one of 87.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 87.0Firefox 88.0 beta 6
accessibility.mouse_focuses_formcontrol false 1
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.enabled false true
apz.mac.enable_double_tap_zoom_touchpad_gesture undefined false
apz.scrollbarbuttonrepeat.enabled undefined true
browser.aboutwelcome.design undefined
browser.aboutwelcome.overrideContent undefined
browser.aboutwelcome.screens undefined
browser.cache.offline.enable true false
browser.defaultbrowser.notificationbar false true
browser.newtabpage.activity-stream.feeds.section.topstories.options {"api_key_pref":"extensions.pocket.oAuthConsumerKey","hidden":true,"provider_icon":"pocket","provider_name":"Pocket","read_more_endpoint":"https://getpocket.com/explore/trending?src=fx_new_tab","stories_endpoint":"https://getpocket.cdn.mozilla.net/v3/firefox/global-recs?version=3&consumer_key=$apiKey&locale_lang=en-US&feed_variant=default_spocs_off","stories_referrer":"https://getpocket.com/recommendations","topics_endpoint":"https://getpocket.cdn.mozilla.net/v3/firefox/trending-topics?version=2&consumer_key=$apiKey&locale_lang=en-US","model_keys":["nmf_model_animals","nmf_model_business","nmf_model_career","nmf_model_datascience","nmf_model_design","nmf_model_education","nmf_model_entertainment","nmf_model_environment","nmf_model_fashion","nmf_model_finance","nmf_model_food","nmf_model_health","nmf_model_home","nmf_model_life","nmf_model_marketing","nmf_model_politics","nmf_model_programming","nmf_model_science","nmf_model_shopping","nmf_model_sports","nmf_model_tech","nmf_model_travel","nb_model_animals","nb_model_books","nb_model_business","nb_model_career","nb_model_datascience","nb_model_design","nb_model_economics","nb_model_education","nb_model_entertainment","nb_model_environment","nb_model_fashion","nb_model_finance","nb_model_food","nb_model_game","nb_model_health","nb_model_history","nb_model_home","nb_model_life","nb_model_marketing","nb_model_military","nb_model_philosophy","nb_model_photography","nb_model_politics","nb_model_productivity","nb_model_programming","nb_model_psychology","nb_model_science","nb_model_shopping","nb_model_society","nb_model_space","nb_model_sports","nb_model_tech","nb_model_travel","nb_model_writing"],"show_spocs":false,"personalized":true,"version":1} {"api_key_pref":"extensions.pocket.oAuthConsumerKey","hidden":true,"provider_icon":"pocket","provider_name":"Pocket","read_more_endpoint":"https://getpocket.com/explore/trending?src=fx_new_tab","stories_endpoint":"https://getpocket.cdn.mozilla.net/v3/firefox/global-recs?version=3&consumer_key=$apiKey&locale_lang=en-US&feed_variant=default_spocs_off","stories_referrer":"https://getpocket.com/recommendations","topics_endpoint":"https://getpocket.cdn.mozilla.net/v3/firefox/trending-topics?version=2&consumer_key=$apiKey&locale_lang=en-US","show_spocs":false}
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.slowStartup.maxSamples 5 undefined
browser.slowStartup.notificationDisabled false undefined
browser.slowStartup.timeThreshold 20000 undefined
browser.tabs.crashReporting.email undefined
browser.tabs.crashReporting.emailMe false undefined
browser.tabs.crashReporting.requestEmail false undefined
browser.tabs.remote.useOriginAttributesInRemoteType false true
devtools.performance.recording.features ["js","leaf","stackwalk","screenshots"] ["js","leaf","stackwalk","cpu","screenshots"]
devtools.performance.recording.features.remote ["js","leaf","stackwalk","screenshots","java"] ["js","leaf","stackwalk","cpu","screenshots","java"]
dom.event.wheel-deltaMode-lines-to-pixel-scale 30 undefined
dom.event.wheel-deltaMode-lines.always-enabled undefined
dom.event.wheel-deltaMode-lines.disabled false true
dom.ipc.processPrelaunch.startupDelayMs undefined 1000
dom.max_script_run_time.require_critical_input undefined true
dom.max_script_run_time_without_important_user_input 10 undefined
dom.placeholder.show_on_focus true undefined
dom.presentation.controller.enabled false undefined
dom.presentation.discoverable false undefined
dom.presentation.discoverable.encrypted true undefined
dom.presentation.discoverable.retry_ms 5000 undefined
dom.presentation.discovery.enabled false undefined
dom.presentation.discovery.timeout_ms 10000 undefined
dom.presentation.enabled false undefined
dom.presentation.receiver.enabled false undefined
dom.presentation.session_transport.data_channel.enable false undefined
dom.presentation.tcp_server.debug false undefined
dom.presentation.testing.simulate-receiver false undefined
dom.security.https_only_mode_break_upgrade_downgrade_endless_loop undefined true
dom.security.https_only_mode_error_page_user_suggestions undefined false
dom.storage.next_gen false true
dom.webdriver.enabled true undefined
dom.webidl.crosscontext_hasinstance.enabled undefined true
dom.window.content.untrusted.enabled true false
dom.window_print.fuzzing.block_while_printing undefined false
dom.workers.testing.enabled undefined false
extensions.webcompat-reporter.enabled false true
fission.bfcacheInParent 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.prefer-x11-egl false undefined
gfx.webrender.software.d3d11.upload-mode 2 4
gfx.webrender.software.opengl undefined false
gfx.webrender.software.unaccelerated-widget.allow undefined false
gfx.webrender.software.unaccelerated-widget.force undefined false
gfx.webrender.unaccelerated-widget.force undefined false
gfx.x11-egl.force-disabled undefined false
gfx.x11-egl.force-enabled undefined false
identity.fxaccounts.useSessionTokensForOAuth true undefined
identity.sync.useOAuthForSyncToken true undefined
image.avif.enabled false true
javascript.options.showInConsole true undefined
javascript.options.spectre.jit_to_C++_calls true undefined
javascript.options.spectre.jit_to_cxx_calls undefined true
javascript.options.spectre.object_mitigations undefined true
javascript.options.spectre.object_mitigations.barriers true undefined
javascript.options.spectre.object_mitigations.misc true undefined
javascript.options.wasm_simd_wormhole undefined false
layers.omtp.enabled true false
layout.css.color-mix.enabled undefined false
layout.css.image-set.enabled false true
layout.css.moz-submit-invalid.enabled undefined false
layout.css.outline-follows-border-radius.enabled false true
layout.css.ruby.position-alternate.enabled undefined true
layout.dynamic-reflow-roots.enabled false true
marionette.contentListener false undefined
marionette.enabled false undefined
media.decoder-doctor.notifications-allowed MediaWMFNeeded,MediaWidevineNoWMF,MediaCannotInitializePulseAudio,MediaCannotPlayNoDecoders,MediaUnsupportedLibavcodec MediaWMFNeeded,MediaWidevineNoWMF,MediaCannotInitializePulseAudio,MediaCannotPlayNoDecoders,MediaUnsupportedLibavcodec,MediaPlatformDecoderNotFound
media.decoder-doctor.testing undefined false
media.peerconnection.mtransport_process false true
network.cache.frecency_array_check_enabled false true
network.ftp.enabled true false
network.http.http3.enable_0rtt undefined true
network.http.http3.enabled false true
network.notify.resolvers undefined true
network.process.enabled false true
network.trr.attempt-when-retrying-confirmation undefined false
network.trr.odoh.configs_uri undefined
network.trr.odoh.min_ttl undefined 60
network.trr.wait-for-confirmation true false
pdfjs.enableScripting false true
privacy.annotate_channels.strict_list.enabled false true
privacy.webrtc.deviceGracePeriodTimeoutMs undefined 50000
prompts.contentPromptSubDialog false true
reader.parse-on-load.force-enabled false undefined
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
signon.formRemovalCapture.enabled undefined true
svg.text-spacing.enabled true undefined
toolkit.telemetry.enabled false true
widget.disable-native-theme-for-content false undefined
widget.dmabuf.force-enabled undefined false
widget.gtk.non-native.round-thumb true undefined
widget.gtk.non-native.scrollbar.allow-buttons false undefined
widget.gtk.non-native.scrollbar.normal-size 12 undefined
widget.gtk.non-native.scrollbar.thin-size 6 undefined
widget.gtk.non-native.scrollbar.thumb-cross-size 40 undefined
widget.gtk.non-native.scrollbar.thumb-size 0.75 undefined
widget.non-native-theme.enabled undefined false
widget.non-native-theme.gtk.scrollbar.allow-buttons undefined false
widget.non-native-theme.gtk.scrollbar.round-thumb undefined true
widget.non-native-theme.gtk.scrollbar.thumb-cross-size undefined 40
widget.non-native-theme.gtk.scrollbar.thumb-size undefined 0.75
widget.non-native-theme.scrollbar.size undefined 12
widget.non-native-theme.use-theme-accent undefined true
widget.non-native-theme.webrender undefined true
widget.non-native-theme.win.scrollbar.use-system-size undefined true
widget.non-native.use-theme-accent true undefined