This article shows the difference between the default preferences of Firefox 156.0 beta 5 and Firefox 155.0.1. The diff table below is generated using my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 155.0.1Firefox 156.0 beta 5
app.releaseNotesURL https://www.firefox.com/%LOCALE%/firefox/%VERSION%/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-desktop&utm_campaign=whatsnew https://www.firefox.com/%LOCALE%/firefox/%VERSION%beta/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-desktop&utm_campaign=whatsnew
app.releaseNotesURL.aboutDialog https://www.firefox.com/%LOCALE%/firefox/%VERSION%/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-desktop&utm_campaign=about-dialog https://www.firefox.com/%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/new?reason=manual-update https://www.mozilla.org/%LOCALE%/firefox/beta?reason=manual-update
browser.autocomplete.removeRecords.enabled undefined false
browser.cache.disk.encryption.key undefined
browser.contentblocking.reject-and-isolate-cookies.preferences.ui.enabled true undefined
browser.ipProtection.hasSeenFeature undefined false
browser.ipProtection.l10nGateVersion undefined 0
browser.ml.linkPreview.smokeTest.lastBuildID undefined
browser.ml.modelHub.testing undefined false
browser.netError.searchCTA.connectivityFreshnessMs undefined 60000
browser.netError.searchCTA.connectivityRecheckTimeoutMs undefined 3000
browser.newtabpage.activity-stream.discoverystream.carousel.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.carousel.paused undefined false
browser.newtabpage.activity-stream.discoverystream.carousel.slideCount undefined 5
browser.newtabpage.activity-stream.discoverystream.dailyBrief.enabled false undefined
browser.newtabpage.activity-stream.discoverystream.dailyBrief.sectionId top_stories_section undefined
browser.newtabpage.activity-stream.discoverystream.sections.ordering default
browser.newtabpage.activity-stream.feeds.recentsearchesfeed undefined true
browser.newtabpage.activity-stream.feeds.sportsfeed true false
browser.newtabpage.activity-stream.newtabWallpapers.visibilityGroups undefined
browser.newtabpage.activity-stream.pageLayouts.variant undefined nova-full-width
browser.newtabpage.activity-stream.privacyInfo.url https://support.mozilla.org/1/firefox/155.0.1/Linux/en-US/sponsor-privacy https://support.mozilla.org/1/firefox/156.0/Linux/en-US/sponsor-privacy
browser.newtabpage.activity-stream.sectionsLearnMore.url https://support.mozilla.org/1/firefox/155.0.1/Linux/en-US/firefox-new-tab-personalization https://support.mozilla.org/1/firefox/156.0/Linux/en-US/firefox-new-tab-personalization
browser.newtabpage.activity-stream.spaces.activityOptOut undefined false
browser.newtabpage.activity-stream.spaces.storiesOptOut undefined false
browser.newtabpage.activity-stream.spaces.widgetsOptOut undefined false
browser.newtabpage.activity-stream.support.url https://support.mozilla.org/1/firefox/155.0.1/Linux/en-US/new-tab https://support.mozilla.org/1/firefox/156.0/Linux/en-US/new-tab
browser.newtabpage.activity-stream.widgets.recentSearches.enabled undefined true
browser.newtabpage.activity-stream.widgets.recentSearches.interaction undefined false
browser.newtabpage.activity-stream.widgets.recentSearches.size undefined
browser.newtabpage.activity-stream.widgets.stocks.watchlist undefined
browser.newtabpage.activity-stream.widgets.system.recentSearches.enabled undefined false
browser.promo.relay.enabled undefined true
browser.smartwindow.agent.toolbar.enabled undefined false
browser.smartwindow.aiTabHistory.logLevel undefined Error
browser.smartwindow.aiTabStore.logLevel undefined Error
browser.smartwindow.aitab.enabled undefined false
browser.smartwindow.aitab.viewerURL undefined
browser.smartwindow.worldcup.enabled false undefined
browser.smartwindow.worldcup.endpointURL https://merino.services.mozilla.com undefined
browser.smartwindow.worldcup.timeoutMs 2000 undefined
browser.urlbar.newtab.featureGate undefined false
browser.urlbar.searchModeSwitcher.skipTabStop undefined false
dom.compression_streams.brotli.large_window.enabled undefined false
dom.enable_container_timing undefined false
dom.media.pseudo-classes.enabled true undefined
dom.range.cut_contents.use_flat_tree undefined false
dom.viewTransitions.enabled true undefined
extensions.formautofill.addresses.supported detect on
extensions.formautofill.creditCards.cvv.enabled undefined false
extensions.formautofill.useml.timeoutMS undefined 120000
extensions.formautofill.useml.twoHead undefined true
extensions.webcompat-reporter.enabled false true
identity.fxaccounts.pairing.version undefined 1
identity.fxaccounts.telemetry.clientInfoPing.enabled undefined true
image.jpeg.dct-scaling.enabled undefined true
image.jpeg.dct-scaling.min-factor undefined 2.5
javascript.options.wasm_baseline_debug undefined false
javascript.options.wasm_cont_stack_max_vmem undefined -2147483648
layout.css.anchor-positioning.follows-transforms.enabled undefined false
layout.css.contrast-color.enabled true undefined
layout.css.gradient-color-interpolation-method.enabled true undefined
layout.css.light-dark.images.enabled true undefined
layout.css.moz-broken.content.enabled false undefined
layout.css.moz-placeholder.content.enabled undefined false
layout.css.relative-color-syntax.enabled true undefined
layout.forms.button-input-align-content-block.enabled undefined false
lightweightThemes.getMoreURL https://addons.mozilla.org/%LOCALE%/firefox/themes undefined
media.hardware-video-decoding-vulkan.force-enabled undefined false
media.navigator.mediadatadecoder_av1_enabled undefined false
network.android_doh.rollout_request_timeout_ms undefined 250
network.http.http3.apple_fast_datapath true false
network.trr.idle_timeout_for_http3_conn 30 undefined
pdfjs.embedFallback undefined true
privacy.partition.serviceWorkers true undefined
remote.experimental.dynamicstart.banner.enabled undefined true
remote.experimental.dynamicstart.connectionbanner.enabled undefined true
remote.experimental.dynamicstart.prompt.enabled undefined true
signon.storage.rust.enabled false true
toolkit.telemetry.enabled false true
widget.use-xdg-desktop-portal.notification undefined 2