This is difference between the default preference of Firefox 138.0 beta 9 and one of 137.0.2. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 137.0.2Firefox 138.0 beta 9
alerts.showFavicons false 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.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.max_connections undefined 32
browser.ml.checkForMemory false true
browser.ml.defaultModelMemoryUsage 1 undefined
browser.ml.enable false true
browser.ml.linkPreview.allowedLanguages undefined en
browser.ml.linkPreview.blockListEnabled undefined true
browser.ml.linkPreview.enabled undefined false
browser.ml.linkPreview.outputSentences undefined 3
browser.ml.maximumMemoryPressure 80 undefined
browser.ml.overridePipelineOptions undefined {}
browser.ml.queueWaitInterval 1 undefined
browser.ml.queueWaitTimeout 60 undefined
browser.newtabpage.activity-stream.discoverystream.reportContent.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.user.enabled undefined true
browser.newtabpage.activity-stream.feeds.newtabmessaging undefined true
browser.newtabpage.activity-stream.newtabAdSize.mediumRectangle undefined false
browser.newtabpage.activity-stream.newtabWallpapers.customWallpaper.fileSize undefined 0
browser.newtabpage.activity-stream.newtabWallpapers.customWallpaper.fileSize.enabled undefined false
browser.newtabpage.activity-stream.newtabWallpapers.customWallpaper.uuid undefined
browser.newtabpage.activity-stream.support.url https://support.mozilla.org/1/firefox/137.0.2/Linux/en-US/new-tab https://support.mozilla.org/1/firefox/138.0/Linux/en-US/new-tab
browser.preonboarding.onTrainRolloutPopulation 100 0
browser.safebrowsing.only_top_level false true
browser.safebrowsing.provider.mozilla.lists base-track-digest256,mozstd-trackwhite-digest256,google-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,ads-track-digest256,social-track-digest256,analytics-track-digest256,base-fingerprinting-track-digest256,content-fingerprinting-track-digest256,base-cryptomining-track-digest256,content-cryptomining-track-digest256,fanboyannoyance-ads-digest256,fanboysocial-ads-digest256,easylist-ads-digest256,easyprivacy-ads-digest256,adguard-ads-digest256,social-tracking-protection-digest256,social-tracking-protection-facebook-digest256,social-tracking-protection-linkedin-digest256,social-tracking-protection-twitter-digest256,base-email-track-digest256,content-email-track-digest256 base-track-digest256,mozstd-trackwhite-digest256,google-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,ads-track-digest256,social-track-digest256,analytics-track-digest256,base-fingerprinting-track-digest256,content-fingerprinting-track-digest256,base-cryptomining-track-digest256,content-cryptomining-track-digest256,fanboyannoyance-ads-digest256,fanboysocial-ads-digest256,easylist-ads-digest256,easyprivacy-ads-digest256,adguard-ads-digest256,social-tracking-protection-digest256,social-tracking-protection-facebook-digest256,social-tracking-protection-linkedin-digest256,social-tracking-protection-twitter-digest256,base-email-track-digest256,content-email-track-digest256,consent-manager-track-digest256
browser.search.rustSelector.featureGate undefined false
browser.shopping.experience2023.autoClose.userEnabled true undefined
browser.shopping.experience2023.shoppingSidebar true undefined
browser.tabs.dragDrop.createGroup.delayMS undefined 240
browser.tabs.dragDrop.expandGroup.delayMS undefined 350
browser.tabs.dragDrop.moveOverThresholdPercent undefined 80
browser.tabs.dragDrop.selectTab.delayMS undefined 350
browser.tabs.dragdrop.moveOverThresholdPercent 80 undefined
browser.tabs.fadeOutExplicitlyUnloadedTabs undefined false
browser.tabs.groups.dragOverDelayMS 240 undefined
browser.tabs.groups.smart.optin undefined false
browser.tabs.groups.smart.userEnabled undefined true
browser.uitour.requireSecure true undefined
browser.urlbar.weather.featureGate false true
devtools.performance.aboutprofiling.has-developer-options undefined false
dom.compression_streams.zstd.enabled undefined false
dom.cookieStore.manager.enabled undefined false
dom.document.name_getter_follow_spec.enabled undefined true
dom.events.mouse-pointer-boundary.keep-enter-targets-after-over-target-removed true undefined
dom.origin_agent_cluster.default undefined false
dom.origin_agent_cluster.enabled undefined true
dom.performance.event_timing.enable_interactionid undefined false
dom.popup.experimental true undefined
dom.quotaManager.groupInitialization.pauseOnIOThreadMs undefined 0
dom.selection.mimic_chrome_tostring.enabled undefined false
dom.serviceWorkers.privateBrowsing.enabled undefined false
dom.storage_access.auto_grants.exclude_third_party_trackers undefined false
dom.timeout.defer_during_load.force-disable undefined *.usps.com, *.nvidia.com, *.fedex.com, *.msci.com
dom.viewTransitions.live-capture undefined true
dom.viewTransitions.wr-old-capture undefined true
dom.webnotifications.actions.enabled undefined false
extensions.formautofill.heuristics.detectDynamicFormChanges undefined true
extensions.formautofill.heuristics.fillOnDynamicFormChanges undefined true
extensions.formautofill.heuristics.fillOnDynamicFormChanges.timeout undefined 1000
extensions.webcompat-reporter.enabled false true
gfx.canvas.remote.use-draw-image-fast-path-d3d false true
gfx.canvas.willreadfrequently.enabled true undefined
javascript.options.experimental.error_capture_stack_trace false true
javascript.options.experimental.error_iserror undefined true
javascript.options.experimental.import_attributes false true
layout.css.details-content.enabled undefined false
media.navigator.mozgetusermedia.enabled undefined true
media.navigator.video.disable_h264_baseline true false
network.dnsCacheEntries 400 800
network.xhr.block_sync_system_requests undefined true
pdfjs.enableSignatureEditor false true
pdfjs.viewerCssTheme undefined 0
privacy.clearSiteDataHeader.cache.enabled false true
privacy.imageInputTelemetry.enableTestMode false undefined
privacy.trackingprotection.consentmanager.annotate_channels undefined true
privacy.trackingprotection.consentmanager.skip.enabled undefined false
privacy.trackingprotection.consentmanager.skip.pbmode.enabled undefined true
security.browser_xhtml_csp.report-only true false
security.client_auth_certificate_default_remember_setting undefined 2
security.remember_cert_checkbox_default_setting true undefined
sidebar.expandOnHover false true
sidebar.main.tools aichat,syncedtabs,history
sidebar.newTool.migration.reviewchecker undefined {"visibilityPref": "browser.shopping.experience2023.integratedSidebar"}
sidebar.revamp.defaultLauncherVisible undefined true
test.currentTimeOffsetSeconds undefined 0
toolkit.crashreporter.infoURL https://www.mozilla.org/legal/privacy/firefox.html#crash-reporter undefined
toolkit.telemetry.enabled false true
urlclassifier.disallow_completions goog-downloadwhite-digest256,base-track-digest256,mozstd-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,ads-track-digest256,social-track-digest256,analytics-track-digest256,base-fingerprinting-track-digest256,content-fingerprinting-track-digest256,base-cryptomining-track-digest256,content-cryptomining-track-digest256,fanboyannoyance-ads-digest256,fanboysocial-ads-digest256,easylist-ads-digest256,easyprivacy-ads-digest256,adguard-ads-digest256,social-tracking-protection-digest256,social-tracking-protection-facebook-digest256,social-tracking-protection-linkedin-digest256,social-tracking-protection-twitter-digest256,base-email-track-digest256,content-email-track-digest256 goog-downloadwhite-digest256,base-track-digest256,mozstd-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,ads-track-digest256,social-track-digest256,analytics-track-digest256,base-fingerprinting-track-digest256,content-fingerprinting-track-digest256,base-cryptomining-track-digest256,content-cryptomining-track-digest256,fanboyannoyance-ads-digest256,fanboysocial-ads-digest256,easylist-ads-digest256,easyprivacy-ads-digest256,adguard-ads-digest256,social-tracking-protection-digest256,social-tracking-protection-facebook-digest256,social-tracking-protection-linkedin-digest256,social-tracking-protection-twitter-digest256,base-email-track-digest256,content-email-track-digest256,consent-manager-track-digest256
urlclassifier.features.consentmanager.annotate.allowlistTables undefined mozstd-trackwhite-digest256
urlclassifier.features.consentmanager.annotate.blocklistTables undefined consent-manager-track-digest256
webextensions.tests false undefined