This is difference between the default preference of Firefox 140.0 beta 9 and one of 139.0.4. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 139.0.4Firefox 140.0 beta 9
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.contextual-services.contextId.rotation-in-days undefined 0
browser.contextual-services.contextId.rust-component.enabled undefined false
browser.ml.linkPreview.collapsed undefined false
browser.ml.linkPreview.ignoreMs undefined 2000
browser.ml.linkPreview.longPress undefined true
browser.ml.linkPreview.longPressMs undefined 1000
browser.ml.linkPreview.optin undefined false
browser.ml.linkPreview.recentTypingMs undefined 1000
browser.ml.linkPreview.shift undefined true
browser.ml.linkPreview.shiftAlt undefined false
browser.ml.minimumPhysicalMemory 4 3
browser.newtabpage.activity-stream.discoverystream.merino-provider.ohttp.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.ohttp.configURL undefined https://prod.ohttp-gateway.prod.webservices.mozgcp.net/ohttp-configs
browser.newtabpage.activity-stream.discoverystream.ohttp.relayURL undefined https://mozilla-ohttp.fastly-edge.com/
browser.newtabpage.activity-stream.discoverystream.placements.contextualSpocs undefined newtab_stories_1, newtab_stories_2, newtab_stories_3, newtab_stories_4, newtab_stories_5, newtab_stories_6
browser.newtabpage.activity-stream.discoverystream.placements.contextualSpocs.counts undefined 1, 1, 1, 1, 1, 1
browser.newtabpage.activity-stream.discoverystream.refinedCardsLayout.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.sections.contextualAds.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.sections.contextualAds.locale-config undefined en-US,en-GB,en-CA
browser.newtabpage.activity-stream.discoverystream.sections.contextualAds.region-config undefined
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.blocked false undefined
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.model.override undefined
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.position 1 undefined
browser.newtabpage.activity-stream.discoverystream.thumbsUpDown.region-thumbs-config US
browser.newtabpage.activity-stream.feeds.inferredpersonalizationfeed undefined true
browser.newtabpage.activity-stream.newtabAdSize.variant-a false undefined
browser.newtabpage.activity-stream.newtabAdSize.variant-b false undefined
browser.newtabpage.activity-stream.newtabWallpapers.customColor.enabled false true
browser.newtabpage.activity-stream.newtabWallpapers.customWallpaper.enabled false true
browser.newtabpage.activity-stream.newtabWallpapers.v2.enabled true undefined
browser.newtabpage.activity-stream.showSponsoredCheckboxes undefined true
browser.newtabpage.activity-stream.support.url https://support.mozilla.org/1/firefox/139.0.4/Linux/en-US/new-tab https://support.mozilla.org/1/firefox/140.0/Linux/en-US/new-tab
browser.newtabpage.activity-stream.system.showSponsoredCheckboxes undefined false
browser.newtabpage.activity-stream.telemetry.privatePing.inferredInterests.enabled undefined false
browser.newtabpage.activity-stream.telemetry.privatePing.maxSubmissionDelayMs undefined 5000
browser.newtabpage.activity-stream.telemetry.privatePing.redactNewtabPing.enabled undefined false
browser.newtabpage.activity-stream.unifiedAds.ohttp.enabled undefined false
browser.search.rustSelector.featureGate false true
browser.shopping.experience2023.active true undefined
browser.shopping.experience2023.ads.enabled false undefined
browser.shopping.experience2023.ads.userEnabled true undefined
browser.shopping.experience2023.autoOpen.enabled false undefined
browser.shopping.experience2023.autoOpen.userEnabled true undefined
browser.shopping.experience2023.enabled false undefined
browser.shopping.experience2023.integratedSidebar false undefined
browser.shopping.experience2023.newPositionCard.hasSeen false undefined
browser.shopping.experience2023.optedIn 0 undefined
browser.shopping.experience2023.showKeepSidebarClosedMessage true undefined
browser.shopping.experience2023.sidebarClosedCount 0 undefined
browser.shopping.experience2023.survey.enabled true undefined
browser.shopping.experience2023.survey.hasSeen false undefined
browser.shopping.experience2023.survey.pdpVisits 0 undefined
browser.tabs.fadeOutExplicitlyUnloadedTabs false true
browser.tabs.unloadTabInContextMenu false true
browser.urlbar.addons.featureGate true false
browser.urlbar.mdn.featureGate undefined false
browser.urlbar.pocket.featureGate false undefined
browser.urlbar.scotchBonnet.enableOverride false true
browser.urlbar.suggest.pocket true undefined
browser.urlbar.suggest.semanticHistory.minLength undefined 5
browser.urlbar.update2.engineAliasRefresh undefined true
browser.urlbar.weather.featureGate true false
browser.urlbar.yelp.featureGate true false
browser.urlbar.yelp.serviceResultDistinction undefined false
devtools.debugger.features.codemirror-next true undefined
dom.cookieStore.enabled false true
dom.cookieStore.extra.enabled undefined false
dom.cookieStore.manager.enabled false true
dom.customHighlightAPI.enabled false true
dom.dialog.light-dismiss.enabled undefined false
dom.element.customstateset.enabled true undefined
dom.element.dialog.toggle_events.enabled true undefined
dom.event.mouse.boundary.restore_last_over_target_from_temporary_removal undefined false
dom.event.pointer.boundary.dispatch_when_layout_change undefined true
dom.event.pointer.rawupdate.enabled undefined true
dom.mutation_events.enabled true false
dom.security.html_serialization_escape_lt_gt false true
dom.serviceWorkers.privateBrowsing.enabled false true
dom.timeout.defer_during_load.force-disable *.usps.com, *.nvidia.com, *.fedex.com, *.msci.com *.usps.com, *.nvidia.com, *.fedex.com, *.msci.com, *.the-saleroom.com
dom.viewTransitions.live-capture true undefined
dom.w3c_pointer_events.dispatch_click_on_pointer_capturing_element undefined true
dom.w3c_pointer_events.dispatch_click_on_pointer_capturing_element.except_touch undefined true
dom.webcomponents.shadowdom.declarative.enabled true undefined
extensions.dataCollectionPermissions.enabled false true
extensions.htmlaboutaddons.local_model_management undefined true
extensions.ml.enabled false true
extensions.pocket.enabled true false
extensions.screenshots.disabled false undefined
extensions.webcompat-reporter.enabled false true
gfx.webrender.debug.highlight-backdrop-filters undefined false
image.natural-size-fallback.enabled undefined false
javascript.options.atomics_wait_async undefined false
javascript.options.experimental.self_hosted_cache undefined false
javascript.options.ion.regalloc undefined 0
javascript.options.mem.nursery_max_time_goal_ms undefined 4
javascript.options.property_error_message_fix false true
javascript.options.warn_asmjs_deprecation undefined true
javascript.options.wasm_lazy_tiering false true
javascript.options.wasm_trace_api undefined false
javascript.options.wasm_unroll_loops false true
layout.css.android-pip.enabled undefined true
layout.css.h1-in-section-ua-styles.enabled true false
layout.css.light-dark.enabled true undefined
layout.css.light-dark.images.enabled undefined false
layout.css.video-dynamic-range.allows-high false true
media.peerconnection.dtls.version.max 771 772
network.android_doh.autoselect_enabled undefined false
network.connectivity-service.wait_for_idle_startup undefined true
network.cookie.chips.partitionLimitDryRun false true
network.dns.always_ai_canonname undefined true
network.dns.native_https_timeout_mac undefined 5000
network.http.http3.ecn false undefined
network.http.http3.ecn_mark undefined false
network.http.http3.ecn_report undefined true
network.sniff.use_extension undefined false
nimbus.profilesdatastoreservice.enabled undefined true
pdfjs.capCanvasAreaFactor undefined 200
places.semanticHistory.featureGate undefined false
remote.experimental-parent-navigation.enabled undefined false
security.csp.reporting.enabled undefined true
security.mixed_content.block_object_subrequest false undefined
security.sandbox.content.headless true undefined
security.sandbox.content.level 4 6
security.sandbox.socket.process.level 1 2
security.xfocsp.errorReporting.automatic false undefined
security.xfocsp.errorReporting.enabled true undefined
sidebar.newTool.migration.reviewchecker {"visibilityPref": "browser.shopping.experience2023.integratedSidebar"} undefined
toolkit.shopping.environment prod undefined
toolkit.shopping.experience2023.defr false undefined
toolkit.telemetry.enabled false true
urlclassifier.delay_prefixes_crc32_check undefined false
widget.gtk.clipboard_timeout_ms undefined 1000