This is difference between the default preference of Firefox 139.0 beta 10 and one of 138.0.4. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 138.0.4Firefox 139.0 beta 10
accessibility.typeaheadfind.autostart true 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.multiSessionInstallLockout.enabled false undefined
app.update.multiSessionInstallLockout.timeoutMs 345600000 undefined
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.contentblocking.features.strict tp,tpPrivate,cookieBehavior5,cookieBehaviorPBM5,cm,fp,stp,emailTP,emailTPPrivate,lvl2,rp,rpTop,ocsp,qps,qpsPBM,fpp,fppPrivate,btp tp,tpPrivate,cookieBehavior5,cookieBehaviorPBM5,cryptoTP,fp,stp,emailTP,emailTPPrivate,-consentmanagerSkip,-consentmanagerSkipPrivate,lvl2,rp,rpTop,ocsp,qps,qpsPBM,fpp,fppPrivate,btp
browser.hiddenWindowChromeURL chrome://browser/content/hiddenWindowMac.xhtml undefined
browser.ml.linkPreview.noKeyPointsRegions undefined AD,AT,BE,BG,CH,CY,CZ,DE,DK,EE,ES,FI,FR,GR,HR,HU,IE,IS,IT,LI,LT,LU,LV,MT,NL,NO,PL,PT,RO,SE,SI,SK
browser.newtabpage.activity-stream.discoverystream.reportAds.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.reportContent.enabled false undefined
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.blocked undefined false
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.locale-config undefined en-US,en-GB,en-CA
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.position undefined 1
browser.newtabpage.activity-stream.discoverystream.sections.personalization.inferred.region-config undefined
browser.newtabpage.activity-stream.feeds.startupcacheinit undefined true
browser.newtabpage.activity-stream.mobileDownloadModal.enabled undefined false
browser.newtabpage.activity-stream.mobileDownloadModal.variant-a undefined false
browser.newtabpage.activity-stream.mobileDownloadModal.variant-b undefined false
browser.newtabpage.activity-stream.mobileDownloadModal.variant-c undefined false
browser.newtabpage.activity-stream.newtabShortcuts.refresh false true
browser.newtabpage.activity-stream.support.url https://support.mozilla.org/1/firefox/138.0.4/Linux/en-US/new-tab https://support.mozilla.org/1/firefox/139.0/Linux/en-US/new-tab
browser.newtabpage.activity-stream.telemetry.privatePing.enabled undefined false
browser.newtabpage.activity-stream.telemetry.surfaceId undefined
browser.preonboarding.onTrainRolloutPopulation 2500 undefined
browser.tabs.groups.smart.embeddingModelRevision undefined latest
browser.tabs.groups.smart.nearestNeighborThresholdInt undefined 275
browser.tabs.groups.smart.suggestOtherTabsMethod undefined NEAREST_NEIGHBOR
browser.tabs.groups.smart.topicModelRevision undefined latest
browser.taskbarTabs.enabled undefined false
browser.theme.colorway-closet true undefined
browser.theme.colorway-migration true undefined
browser.urlbar.perplexity.hasBeenInSearchMode undefined false
browser.urlbar.quicksuggest.ampTopPickCharThreshold 0 5
browser.urlbar.quicksuggest.dynamicSuggestionTypes undefined
browser.urlbar.quicksuggest.exposureSuggestionTypes undefined
devtools.netmonitor.requestfilter undefined
devtools.performance.recording.child.timeout_s 0 15
devtools.webconsole.input.context true undefined
docshell.shistory.sameDocumentNavigationOverridesLoadType undefined true
docshell.shistory.sameDocumentNavigationOverridesLoadType.forceDisable undefined
dom.animations.commit-styles-endpoint-inclusive undefined false
dom.element.contenteditable.plaintext-only.enabled true undefined
dom.element.dialog.request_close.enabled undefined true
dom.events.script_execute.enabled undefined true
dom.hidden_until_found.enabled undefined true
dom.maxHardwareConcurrency 16 128
dom.quotaManager.temporaryStorage.updateOriginAccessTime undefined true
dom.security.html_serialization_escape_lt_gt undefined false
dom.selection.mimic_chrome_tostring.enabled false true
dom.serviceWorkers.privateBrowsing.enabled false true
dom.webgpu.indirect-draw.enabled false undefined
dom.workers.timeoutmanager false true
editor.white_space_normalization.blink_compatible false true
extensions.dataCollectionPermissions.enabled undefined false
extensions.formautofill.heuristics.refillOnSiteClearingFields undefined true
extensions.formautofill.heuristics.refillOnSiteClearingFields.timeout undefined 500
extensions.webcompat-reporter.enabled false true
identity.fxaccounts.toolbar.syncSetup.enabled false undefined
javascript.options.experimental.temporal undefined true
javascript.options.wasm_unroll_factor undefined 3
javascript.options.wasm_unroll_loops undefined false
layout.css.allow-mixed-page-sizes true undefined
layout.css.grid-flex-spanning-items-intrinsic-sizing.enabled false true
layout.css.grid-multi-pass-track-sizing.enabled undefined false
layout.css.page-orientation.enabled true undefined
layout.css.prefixes.transitions false true
layout.overflow-underflow.content.enabled false undefined
layout.overflow-underflow.content.enabled_in_addons false undefined
layout.skip_ticks_while_page_suspended true undefined
network.cookie.chips.partitionLimitDryRun true false
network.http.http2.allow-push false undefined
network.http.http3.max_accumlated_time_ms 1 undefined
network.http.http3.pr_poll_write undefined true
network.proxy.dhcp_wpad_only_one_outstanding undefined true
network.proxy.dhcp_wpad_timeout_sec undefined 5.0
privacy.antitracking.isolateContentScriptResources undefined false
privacy.baselineFingerprintingProtection undefined true
privacy.baselineFingerprintingProtection.granularOverrides undefined
privacy.baselineFingerprintingProtection.overrides undefined
privacy.resistFingerprinting.randomization.canvas.disable_for_chrome false undefined
remote.events.async.enabled true undefined
security.allow_unsafe_dangerous_privileged_evil_eval undefined false
security.osclientcerts.assume_rsa_pss_support true undefined
svg.discard.enabled false undefined
toolkit.aboutLogging.uploadProfileToCloud undefined false
toolkit.telemetry.enabled false true
ui.touch.radius.single_touch.treat_clickable_as_touchable undefined true
urlclassifier.trackingAnnotationSkipURLs google.com/recaptcha/,*.google.com/recaptcha/,d3vox9szr7t2nm.cloudfront.net
webgl.porting.strict_readpixels_formats undefined false
webgl.porting.strict_readpixels_formats.non_es undefined false