This is difference between the default preference of Firefox 136.0 beta 6 and one of 135.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 135.0Firefox 136.0 beta 6
accessibility.ARIAElementReflection.enabled false true
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.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
apz.mousemove_hittest_optimization.enabled false true
apz.popups_without_remote.enabled false true
browser.aboutwelcome.showModal false undefined
browser.disable_pickers_background_tabs undefined true
browser.http.blank_page_with_error_response.enabled undefined true
browser.ml.chat.enabled false true
browser.newtabpage.activity-stream.discoverystream.sections.cards.enabled false true
browser.newtabpage.activity-stream.discoverystream.sections.customizeMenuPanel.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.sections.topicSelection.enabled undefined false
browser.newtabpage.activity-stream.discoverystream.thumbsUpDown.region-thumbs-config US, CA
browser.newtabpage.activity-stream.feeds.adsfeed undefined true
browser.newtabpage.activity-stream.newNewtabExperience.colors #0090ED,#FF4F5F,#2AC3A2,#FF7139,#A172FF,#FFA437,#FF2A8A #004CA4,#009E97,#7550C2,#B63B39,#C96A00,#CA9600,#CC527F
browser.newtabpage.activity-stream.newtabLayouts.variant-b false true
browser.newtabpage.activity-stream.newtabLogo.aprilfools undefined false
browser.newtabpage.activity-stream.newtabShortcuts.refresh undefined false
browser.newtabpage.activity-stream.newtabWallpapers.customColor.enabled undefined false
browser.newtabpage.activity-stream.support.url https://support.mozilla.org/1/firefox/135.0/Linux/en-US/new-tab https://support.mozilla.org/1/firefox/136.0/Linux/en-US/new-tab
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.newtabpage.activity-stream.unifiedAds.adsFeed.enabled undefined false
browser.newtabpage.activity-stream.unifiedAds.adsFeed.spocs.enabled undefined false
browser.newtabpage.activity-stream.unifiedAds.adsFeed.tiles.enabled undefined false
browser.sessionstore.log.appender.file.logOnSuccess false true
browser.sessionstore.loglevel Warn Debug
browser.tabs.groups.dragOverDelayMS 30 120
browser.urlbar.deduplication.thresholdDays 7 0
browser.urlbar.quicksuggest.ampMatchingStrategy undefined 0
browser.urlbar.scotchBonnet.enableOverride false true
captchadetection.actor.enabled false true
datareporting.usage.uploadEnabled undefined true
devtools.debugger.features.codemirror-next false true
devtools.debugger.features.windowless-service-workers false true
devtools.netmonitor.ui.default-raw-response undefined false
dom.cookieStore.enabled false true
dom.document.edit_command.contentReadOnly.enabled true false
dom.document.edit_command.insertBrOnReturn.enabled true false
dom.element.contenteditable.plaintext-only.enabled false true
dom.event.keypress.key.allow_lone_surrogate true false
dom.forms.autocorrect false true
dom.input_events.dispatch_before_compositionend undefined true
dom.origin-trials.mls.state undefined 0
dom.quotaManager.temporaryStorage.incrementalOriginInitialization undefined false
dom.security.https_first false true
dom.security.https_first_for_custom_ports true false
dom.security.https_first_for_local_addresses undefined false
dom.security.https_first_for_unknown_suffixes undefined false
dom.security.https_only_fire_http_request_background_timer_ms 3000 1500
dom.svg.pathSegment.enabled undefined false
dom.text_fragments.create_text_fragment.enabled undefined false
dom.webcomponents.lift-adoptedstylesheets-restriction.enabled undefined false
dom.webgpu.allow-present-without-readback false true
dom.workers.timeoutmanager undefined false
extensions.InstallTrigger.enabled true false
extensions.userScripts.mv3.enabled undefined true
extensions.webcompat-reporter.enabled false true
extensions.webcompat.smartblockEmbeds.enabled false true
font.name-list.emoji Twemoji Mozilla Noto Color Emoji, Twemoji Mozilla
image.exif-density-correction.enabled true undefined
javascript.options.experimental.error_capture_stack_trace undefined false
layout.css.computed-style.dont-resolve-image-local-refs true undefined
layout.css.grid-flex-spanning-items-intrinsic-sizing.enabled undefined false
layout.css.has-slotted-selector.enabled false true
layout.css.hyphenate-limit-chars.enabled undefined false
layout.css.letter-spacing.model 0 2
layout.dynamic-reflow-roots.enabled false true
layout.testing.top-level-always-active undefined false
media.peerconnection.video.use_dd undefined true
media.webrtc.codec.video.av1.enabled false true
media.webrtc.enable_pq_dtls false true
media.webrtc.simulcast.av1.enabled undefined true
media.webrtc.simulcast.h264.enabled undefined true
media.webrtc.simulcast.vp9.enabled false true
memory.lazypurge.enable undefined true
memory.lazypurge.maximum_delay undefined 5000
memory.lazypurge.minimum_idle_budget undefined 5
network.auth.use_redirect_for_retries false true
network.dns.https_rr.check_record_with_cname false true
network.http.basic_http_auth.enabled undefined true
network.http.http2.move_to_pending_list_after_network_change true undefined
network.http.http3.retry_different_ip_family false true
network.http.http3.use_nspr_for_io true false
network.http.move_to_pending_list_after_network_change undefined true
network.http.referer.sendFromRefresh false true
network.http.retry_with_another_half_open false true
network.jar.record_failure_reason false true
network.predictor.enable-prefetch false true
network.psl.onUpdate_notify false undefined
network.socket.ip_addr_any.disabled false true
network.url.max-length 33554432 536870912
network.url.strict_data_url_base64_placement true undefined
pdfjs.enableHighlightFloatingButton false true
permissions.manager.remote.enabled false true
privacy.clearHistory.browsingHistoryAndDownloads undefined true
privacy.clearHistory.formdata undefined false
privacy.clearOnShutdown_v2.browsingHistoryAndDownloads undefined true
privacy.clearOnShutdown_v2.formdata undefined false
privacy.clearSiteData.browsingHistoryAndDownloads undefined false
privacy.clearSiteData.formdata undefined false
privacy.clearSiteDataHeader.cache.enabled undefined false
privacy.resistFingerprinting.randomization.canvas.disable_for_chrome undefined false
privacy.resistFingerprinting.skipEarlyBlankFirstPaint undefined true
privacy.restrict3rdpartystorage.heuristic.exclude_third_party_trackers undefined false
privacy.restrict3rdpartystorage.heuristic.navigation undefined false
privacy.sanitize.clearOnShutdown.hasMigratedToNewPrefs3 undefined false
privacy.sanitize.cpd.hasMigratedToNewPrefs3 undefined false
security.browser_xhtml_csp.enabled undefined true
security.disallow_non_local_systemprincipal_in_tests undefined false
security.insecure_connection_text.enabled false true
security.insecure_connection_text.pbmode.enabled false true
security.mls.enabled false undefined
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 true undefined
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
services.sync.prefs.sync.privacy.clearOnShutdown_v2.browsingHistoryAndDownloads undefined true
services.sync.prefs.sync.privacy.clearOnShutdown_v2.formdata undefined true
signon.rememberSignons.visibilityToggle true undefined
svg.discard.enabled undefined false
toolkit.telemetry.enabled false true
ui.new-webcompat-reporter.send-more-info-link false true