This article shows the difference between the default preferences of Firefox 158.0 beta 3 and Firefox 157.0. The diff table below is generated using my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 157.0Firefox 158.0 beta 3
accessibility.tagged_pdf_output.enabled undefined true
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.contentanalysis.interception_point.clipboard_copy.enabled undefined false
browser.contentanalysis.interception_point.clipboard_copy.plain_text_only undefined true
browser.mini-window.enabled undefined false
browser.mini-window.log undefined false
browser.ml.chat.shortcuts.debounce undefined 200
browser.ml.onnxNativeAvailabilityReported false undefined
browser.netError.illustration.enabled undefined true
browser.newtabpage.activity-stream.discoverystream.endpointSpocsClear https://spocs.getpocket.com/user undefined
browser.newtabpage.activity-stream.discoverystream.endpoints https://getpocket.cdn.mozilla.net/,https://firefox-api-proxy.cdn.mozilla.net/,https://spocs.getpocket.com/,https://merino.services.mozilla.com/,https://ads.mozilla.org/ https://getpocket.cdn.mozilla.net/,https://firefox-api-proxy.cdn.mozilla.net/,https://merino.services.mozilla.com/,https://ads.mozilla.org/
browser.newtabpage.activity-stream.discoverystream.sections.locale-content-config en-US,en-CA,en-GB en-US,en-CA,en-GB,de,fr,it,es-ES,pl
browser.newtabpage.activity-stream.discoverystream.sections.region-content-config US,GB,CA,IE US,GB,CA,IE,IN,DE,AT,CH,BE,FR,IT,ES,PL
browser.newtabpage.activity-stream.newtabWallpapers.customWallpaper.library.enabled false true
browser.newtabpage.activity-stream.pageLayouts.spacesConfig undefined {"order":["games","home","living"],"default":"home","spaces":{"home":{"label":"Home","icon":"chrome://browser/skin/home.svg","sections":["top_stories_section","headlines","in-the-zeitgeist","by-the-numbers","government","business","finance","explained","in-conversation","education-science","tech"],"widgets":["pictureOfTheDay"]},"games":{"label":"Games & Entertainment","icon":"chrome://browser/content/profiles/assets/16_video-game-controller.svg","sections":["arts","hobbies","music","movies","tv","featured-videos","photojournalism","sports","nfl","nhl","mlb","nba","soccer"],"widgets":["crossword"]},"living":{"label":"Living","icon":"chrome://browser/content/profiles/assets/16_leaf.svg","sections":["food","home","travel","society-parenting","health","small-wins","education","long-reads","connections"],"widgets":["lists","focusTimer"]}}}
browser.newtabpage.activity-stream.privacyInfo.url https://support.mozilla.org/1/firefox/157.0/Linux/en-US/sponsor-privacy https://support.mozilla.org/1/firefox/158.0/Linux/en-US/sponsor-privacy
browser.newtabpage.activity-stream.sectionsLearnMore.url https://support.mozilla.org/1/firefox/157.0/Linux/en-US/firefox-new-tab-personalization https://support.mozilla.org/1/firefox/158.0/Linux/en-US/firefox-new-tab-personalization
browser.newtabpage.activity-stream.support.url https://support.mozilla.org/1/firefox/157.0/Linux/en-US/new-tab https://support.mozilla.org/1/firefox/158.0/Linux/en-US/new-tab
browser.newtabpage.activity-stream.topSitesGroupedPins false true
browser.pageextractor.youtube.enabled false true
browser.partnerlink.attributionURL https://topsites.services.mozilla.com/cid/ undefined
browser.partnerlink.campaign.topsites amzn_2020_a1 undefined
browser.privateWindowRedesign.enabled undefined false
browser.privatebrowsing.introAnimationShown undefined false
browser.shell.desktop-entry-api undefined default
browser.smartwindow.agent.expiry.maxAgeDays undefined 90
browser.smartwindow.agent.expiry.noMatchDays undefined 60
browser.smartwindow.agent.toolbar.enabled false true
browser.smartwindow.autoTabGrouping.minCandidateTabs 4 2
browser.smartwindow.resumeCards.enabled undefined false
browser.smartwindow.smartformfill.enabled false true
browser.smartwindow.smartformfill.minFormFields undefined 4
browser.urlbar.newtab.variantA undefined false
browser.urlbar.newtab.variantB undefined false
docshell.shistory.restoreSubframesOnReload undefined true
dom.document.domcontentloaded.synchronous.enabled undefined true
dom.fetch.streaming_upload undefined false
dom.ipc.cpow.timeout 500 undefined
dom.ipc.reply.timeout undefined 500
dom.performance.deliverytype.enabled false true
dom.pointer-lock.reset-to-center-from-parent.enabled true undefined
dom.pointer-lock.unadjusted-movement.enabled true undefined
dom.push.declarative.enabled undefined false
dom.reporting.featurePolicy.enabled false undefined
dom.reporting.permissionsPolicy.enabled undefined false
dom.security.featurePolicy.experimental.enabled false undefined
dom.security.featurePolicy.header.enabled false undefined
dom.security.featurePolicy.webidl.enabled false undefined
dom.security.permissionsPolicy.experimental.enabled undefined false
dom.security.permissionsPolicy.header.enabled undefined false
dom.security.permissionsPolicy.webidl.enabled undefined false
dom.speculation_rules.eager_hover_delay_ms undefined 10
dom.speculation_rules.moderate_hover_delay_ms undefined 200
dom.webnotifications.allowcrossoriginiframe false undefined
dom.webnotifications.navigate.enabled false true
extensions.formautofill.addresses.supported detect on
extensions.formautofill.creditCards.cvv.enabled false true
extensions.formautofill.creditCards.cvv.supported undefined off
extensions.formautofill.creditCards.storage.rust.active undefined false
extensions.formautofill.creditCards.storage.rust.enabled undefined false
extensions.formautofill.creditCards.storage.rust.migrationAttempts undefined 0
extensions.formautofill.creditCards.storage.rust.migrationTestVersion undefined 0
extensions.formautofill.creditCards.storage.rust.runMigrationTest undefined false
extensions.webcompat-reporter.enabled false true
gfx.webrender.blob.largeish-px undefined 512
gfx.webrender.blob.relaxed-active-threshold undefined false
identity.fxaccounts.pairing.version 1 2
identity.fxaccounts.toolbar.appMenuSignInPromo.dismissed undefined false
image.jxl.enabled false true
javascript.options.experimental.temporal true undefined
layers.gpu-process.allow-headless undefined false
layout.css.calc-typed-arithmetic.enabled false true
layout.css.grid-intrinsic-sizing.measure-bsize undefined false
layout.css.quirks.final-uri-check undefined true
layout.css.random.enabled undefined false
layout.css.scroll-anchoring.reset-heuristic-during-animation false undefined
layout.disable-pixel-alignment false true
media.peerconnection.description.legacy.enabled true undefined
media.webrtc.capture.apply-rotation undefined true
media.webrtc.send_mlkem_keyshare true false
media.webspeech.recognition.endpoint_blank_ms undefined 800
network.dns.mru_to_tail false true
network.dnsNegativeCacheExpiration undefined 60
network.dnsNegativeCacheExpirationGracePeriod undefined 60
network.ipc.reparse_deserialized_uri undefined true
network.socket.shutdown_process_events_ms undefined 3000
print.experimental.skpdf undefined true
print.skpdf.image-encoding-quality undefined 101
privacy.trackingprotection.content.mirror.enabled false undefined
privacy.trackingprotection.content.mirror.mode undefined 0
security.connection_allowlists.enabled undefined false
services.sync.perDeviceEngineChoices undefined false
signon.storage.rust.enabled false true
toolkit.asyncshutdown.crash_timeout_additional_wait undefined 10000
toolkit.telemetry.enabled false true
widget.wayland.session-management.enabled false true