This is difference between the default preference of Firefox 127.0 beta 6 and one of 126.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 126.0Firefox 127.0 beta 6
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-desktop&utm_campaign=whatsnew https://www.mozilla.org/%LOCALE%/firefox/%VERSION%beta/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-desktop&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.interval 43200 21600
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.backup.preferences.ui.enabled undefined false
browser.backup.sqlite.pages_per_step undefined 5
browser.backup.sqlite.step_delay_ms undefined 250
browser.contentanalysis.bypass_for_same_tab_operations undefined false
browser.contentanalysis.default_allow false undefined
browser.contentanalysis.default_result undefined 0
browser.ml.modelCacheTimeout undefined 120000
browser.ml.modelHubRootUrl undefined https://model-hub.mozilla.org/
browser.ml.modelHubUrlTemplate undefined {model}/{revision}
browser.newtabpage.activity-stream.discoverystream.contile-topsites-positions 0,1 0,1,2
browser.newtabpage.activity-stream.discoverystream.pocket-feed-parameters undefined
browser.newtabpage.activity-stream.feeds.weatherfeed undefined true
browser.newtabpage.activity-stream.showWeather undefined true
browser.newtabpage.activity-stream.system.showWeather undefined false
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.newtabpage.activity-stream.weather.display undefined simple
browser.newtabpage.activity-stream.weather.locationSearchEnabled undefined false
browser.newtabpage.activity-stream.weather.query undefined
browser.newtabpage.activity-stream.weather.temperatureUnits undefined f
browser.privateWindowSeparation.enabled true undefined
browser.search.serpEventTelemetry.enabled true undefined
browser.sessionstore.log.appender.console undefined Fatal
browser.sessionstore.log.appender.dump undefined Error
browser.sessionstore.log.appender.file.level undefined Trace
browser.sessionstore.log.appender.file.logOnError undefined true
browser.sessionstore.log.appender.file.logOnSuccess undefined true
browser.sessionstore.log.appender.file.maxErrorAge undefined 864000
browser.sessionstore.loglevel undefined Debug
browser.tabs.context.close-duplicate.enabled undefined true
browser.tabs.firefox-view true undefined
browser.tabs.firefox-view-newIcon true undefined
browser.tabs.firefox-view-next true undefined
browser.tabs.firefox-view.notify-for-tabs false undefined
browser.translations.select.enable false true
browser.urlbar.secondaryActions.featureGate undefined false
browser.urlbar.showSearchTerms.featureGate false true
browser.urlbar.trending.featureGate undefined true
browser.urlbar.untrimOnUserInteraction.featureGate undefined false
cookiebanners.ui.desktop.enabled false true
devtools.performance.recording.features ["js","stackwalk","cpu","screenshots"] ["js","stackwalk","cpu","screenshots","memory"]
devtools.performance.recording.features.remote ["js","stackwalk","cpu","screenshots","java"] ["js","stackwalk","cpu","screenshots","memory","java"]
dom.document.edit_command.contentReadOnly.enabled true false
dom.document.edit_command.insertBrOnReturn.enabled true false
dom.event.keypress.key.allow_lone_surrogate true false
dom.events.asyncClipboard.clipboardItem false true
dom.events.mouse-pointer-boundary.keep-enter-targets-after-over-target-removed false true
dom.forms.always_allow_key_and_focus_events.enabled false true
dom.forms.fieldset_disable_only_descendants.enabled false true
dom.memory.memory_pressure_on_background undefined 0
dom.prefetch_dns_for_anchor_http_document undefined true
dom.prefetch_dns_for_anchor_https_document undefined false
dom.security.https_first_schemeless false true
extensions.InstallTrigger.enabled true false
extensions.abuseReport.amWebAPI.enabled true undefined
extensions.abuseReport.amoDetailsURL https://services.addons.mozilla.org/api/v4/addons/addon/ undefined
extensions.abuseReport.amoFormEnabled true undefined
extensions.abuseReport.url https://services.addons.mozilla.org/api/v4/abuse/report/addon/ undefined
extensions.originControls.grantByDefault undefined true
extensions.webcompat-reporter.enabled false true
fission.enforceBlocklistedPrefsInSubprocesses false true
identity.fxaccounts.commands.remoteTabManagement.enabled undefined false
image.decode-sync.enabled false undefined
image.testing.decode-sync.enabled undefined false
javascript.options.experimental.new_set_methods undefined true
javascript.options.mem.gc_max_parallel_marking_threads undefined 2
javascript.options.mem.nursery.max_kb 16384 65536
javascript.options.use_fdlibm_for_sin_cos_tan false true
javascript.options.wasm_branch_hinting undefined false
javascript.options.wasm_exnref false true
javascript.options.wasm_js_promise_integration undefined false
layers.force-shmem-tiles false undefined
layout.css.content.alt-text.enabled undefined false
layout.css.gradient-color-interpolation-method.enabled false true
layout.dynamic-reflow-roots.enabled false true
media.getusermedia.aec 1 undefined
media.getusermedia.aec_enabled true undefined
media.getusermedia.agc 1 undefined
media.getusermedia.agc2_forced true undefined
media.getusermedia.agc_enabled true undefined
media.getusermedia.audio.capture.enabled undefined false
media.getusermedia.audio.max_channels undefined 0
media.getusermedia.audio.processing.aec undefined 1
media.getusermedia.audio.processing.aec.enabled undefined true
media.getusermedia.audio.processing.aec.mobile undefined false
media.getusermedia.audio.processing.agc undefined 1
media.getusermedia.audio.processing.agc.enabled undefined true
media.getusermedia.audio.processing.agc2.forced undefined true
media.getusermedia.audio.processing.hpf.enabled undefined true
media.getusermedia.audio.processing.noise undefined 2
media.getusermedia.audio.processing.noise.enabled undefined true
media.getusermedia.audio.processing.platform.enabled undefined false
media.getusermedia.audio.processing.transient.enabled undefined true
media.getusermedia.audiocapture.enabled false undefined
media.getusermedia.channels 0 undefined
media.getusermedia.hpf_enabled true undefined
media.getusermedia.microphone.voice_stream_priming.enabled undefined false
media.getusermedia.noise 2 undefined
media.getusermedia.noise_enabled true undefined
media.getusermedia.transient_enabled true undefined
media.getusermedia.use_aec_mobile false undefined
media.mediasource.webm.audio.enabled true undefined
media.peerconnection.dtls.version.max 771 772
media.prefer-non-ffvpx undefined false
memory.phc.enabled false true
network.auth.use_redirect_for_retries false true
network.connectivity-service.DNS_HTTPS.domain undefined cloudflare-dns.com
network.cookie.sameSite.noneRequiresSecure false true
network.dns.disablePrefetchFromHTTPS true false
network.dns.native_https_query false true
network.dns.only_refresh_on_fresh_connection undefined true
network.dns.prefetch_via_proxy undefined false
network.http.http3.retry_different_ip_family false true
network.jar.record_failure_reason false true
network.predictor.enable-prefetch false true
network.protocol-handler.external.firefox-bridge false undefined
network.protocol-handler.external.firefox-private-bridge false undefined
network.send_OnDataFinished.cssLoader false true
pdfjs.enableHighlightFloatingButton false true
places.history.floodingPrevention.enabled undefined false
places.history.floodingPrevention.maxSecondsFromLastUserInteraction undefined 3
places.history.floodingPrevention.restrictionCount undefined 3
places.history.floodingPrevention.restrictionExpireSeconds undefined 5
plugins.rewrite_youtube_embeds true false
print.save_as_pdf.internal_destinations.enabled false true
privacy.annotate_channels.strict_list.enabled false true
privacy.bounceTrackingProtection.enableDryRunMode undefined true
privacy.bounceTrackingProtection.hasMigratedUserActivationData undefined false
privacy.bounceTrackingProtection.requireStatefulBounces false true
privacy.fingerprintingProtection.remoteOverrides.enabled undefined true
reader.character_spacing undefined
reader.improved_text_menu.enabled undefined false
reader.text_alignment undefined start
reader.word_spacing undefined
screenshots.browser.component.enabled false true
security.disallow_non_local_systemprincipal_in_tests undefined false
security.mixed_content.upgrade_display_content false true
security.mixed_content.upgrade_display_content.image false true
security.remote_settings.crlite_filters.enabled false true
security.ssl3.ecdhe_ecdsa_aes_128_sha true false
security.ssl3.ecdhe_ecdsa_aes_256_sha true false
security.xfocsp.hideOpenInNewWindow undefined true
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
svg.nearestAndFarthestViewportElement.enabled true false
toolkit.contentRelevancy.ingestEnabled undefined false
toolkit.scrollbox.pagescroll.maxOverlapLines undefined 2
toolkit.scrollbox.pagescroll.maxOverlapPercent undefined 10
toolkit.telemetry.enabled false true
ui.new-webcompat-reporter.send-more-info-link false true
widget.non-native-theme.enabled true undefined
widget.non-native-theme.webrender true undefined