This is difference between the default preference of Firefox 128.0 beta 3 and one of 127.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 127.0Firefox 128.0 beta 3
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
browser.backup.scheduled.enabled undefined false
browser.contentanalysis.allow_url_regex_list ^about:(?!blank|srcdoc).*
browser.display.use_document_fonts.icon_font_allowlist Material Icons, Material Icons Extended, Material Icons Outlined, Material Icons Round, Material Icons Sharp, Material Icons Two Tone, Google Material Icons, Material Symbols Outlined, Material Symbols Round, Material Symbols Rounded, Material Symbols Sharp Material Icons, Material Icons Extended, Material Icons Outlined, Material Icons Round, Material Icons Sharp, Material Icons Two Tone, Google Material Icons, Google Material Icons Filled, Material Symbols Outlined, Material Symbols Round, Material Symbols Rounded, Material Symbols Sharp
browser.firefox-view.search.enabled true undefined
browser.mailto.dualPrompt.dismissNotNowMinutes undefined 525600
browser.mailto.dualPrompt.dismissXClickMinutes undefined 1440
browser.mailto.dualPrompt.onLocationChange undefined false
browser.mailto.prompt.os true undefined
browser.ml.chat.enabled undefined false
browser.ml.chat.prompt.prefix undefined I’m on page "%currentTabTitle%" with "%selection|12000%" selected.
browser.ml.chat.prompts.0 undefined {"label":"Summarize","value":"Please summarize the selection using precise and concise language. Highlight the main themes and conclusions. Use headers and bulleted lists in the summary, to make it scannable. Maintain the meaning of the selection."}
browser.ml.chat.prompts.1 undefined {"label":"Simplify language","value":"Please rewrite the selection in plain, clear language suitable for a general audience without specialized knowledge. Use all of the following tactics: simple vocabulary; short sentences; active voice; examples where applicable to make explanations clearer; explanations for jargon and technical terms; headers and bulleted lists for scannability. Maintain factual accuracy while simplifying."}
browser.ml.chat.prompts.2 undefined {"label":"Quiz me","value":"Please create questions related to the selection. Ask the questions one by one. Wait for my response before moving on to the next question. Evaluate each response. Ask a variety of types of questions, like multiple choice, true or false and short answer."}
browser.ml.chat.provider undefined
browser.newtabpage.activity-stream.newtabWallpapers.highlightContentText undefined
browser.newtabpage.activity-stream.newtabWallpapers.highlightCtaText undefined
browser.newtabpage.activity-stream.newtabWallpapers.highlightDismissed undefined false
browser.newtabpage.activity-stream.newtabWallpapers.highlightEnabled undefined false
browser.newtabpage.activity-stream.newtabWallpapers.highlightHeaderText undefined
browser.newtabpage.activity-stream.newtabWallpapers.highlightSeenCounter undefined 0
browser.newtabpage.activity-stream.newtabWallpapers.v2.enabled undefined false
browser.newtabpage.activity-stream.newtabWallpapers.wallpaper-color undefined
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.search.newSearchConfig.enabled false undefined
browser.sessionstore.log.appender.file.logOnSuccess false true
browser.sessionstore.loglevel Warn Debug
browser.spin_cursor_while_busy undefined false
browser.tabs.allow_transparent_browser undefined false
browser.tabs.cardPreview.enabled false undefined
browser.tabs.cardPreview.showThumbnails true undefined
browser.tabs.hoverPreview.enabled undefined false
browser.tabs.hoverPreview.showThumbnails undefined true
browser.topsites.component.enabled undefined false
browser.translations.select.enable false true
browser.urlbar.contextualSearch.enabled false true
browser.urlbar.recentsearches.featureGate undefined true
browser.urlbar.scotchBonnet.enableOverride undefined false
browser.urlbar.secondaryActions.featureGate false undefined
browser.urlbar.showSearchTerms.featureGate false true
browser.urlbar.trending.featureGate undefined true
browser.urlbar.trending.requireSearchMode undefined false
browser.urlbar.update2.emptySearchBehavior 0 undefined
cookiebanners.ui.desktop.enabled false true
dom.animations-api.compositing.enabled true undefined
dom.animations-api.timelines.enabled true undefined
dom.document.edit_command.contentReadOnly.enabled true false
dom.document.edit_command.insertBrOnReturn.enabled true false
dom.document.exec_command.nested_calls_allowed false undefined
dom.document_priority.incremental undefined true
dom.event.keypress.key.allow_lone_surrogate true false
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.forms.number.hide_spin_buttons_when_no_hover_or_focus false undefined
dom.fullscreen.force_exit_on_multiple_escape_interval undefined 500
dom.mutation-events.cssom.disabled true undefined
dom.no_unknown_protocol_error.enabled true undefined
dom.origin-trials.private-attribution.state undefined 0
dom.private-attribution.submission.enabled undefined true
dom.security.credentialmanagement.identity.heavyweight.enabled undefined false
dom.security.credentialmanagement.identity.lightweight.enabled undefined false
dom.security.https_first_add_exception_on_failiure undefined true
dom.security.https_first_schemeless false true
dom.window.event.enabled true undefined
extensions.InstallTrigger.enabled true false
extensions.abuseReport.amoFormURL https://addons.mozilla.org/%LOCALE%/%APP%/feedback/addon/%addonID%/ https://addons.mozilla.org/%LOCALE%/firefox/feedback/addon/%addonID%/
extensions.addonAbuseReport.url undefined https://services.addons.mozilla.org/api/v5/abuse/report/addon/
extensions.blocklist.addonItemURL https://addons.mozilla.org/%LOCALE%/%APP%/blocked-addon/%addonID%/%addonVersion%/ https://addons.mozilla.org/%LOCALE%/firefox/blocked-addon/%addonID%/%addonVersion%/
extensions.formautofill.heuristics.interactivityCheckMode focusability undefined
extensions.script_blob_without_match_origin_as_fallback undefined false
extensions.webcompat-reporter.enabled false true
fission.enforceBlocklistedPrefsInSubprocesses false true
gfx.canvas.remote.recycle-used-data-surface undefined false
gfx.remote-texture.wait-owner-at-image-host undefined true
identity.fxaccounts.toolbar.pxiToolbarEnabled false true
image.priority.incremental undefined true
io.activity.enabled false undefined
javascript.options.experimental.arraybuffer_resizable undefined true
javascript.options.experimental.sharedarraybuffer_growable undefined true
javascript.options.mem.gc_incremental_slice_ms 5 10
javascript.options.use_fdlibm_for_sin_cos_tan false true
javascript.options.wasm_exnref false true
layout.css.anchor-positioning.enabled undefined false
layout.css.computed-style.shorthands true undefined
layout.css.container-queries.enabled true undefined
layout.css.content.alt-text.enabled false true
layout.css.individual-transform.enabled true undefined
layout.css.properties-and-values.enabled false true
layout.css.relative-color-syntax.enabled false true
layout.dynamic-reflow-roots.enabled false true
layout.forms.textarea-sizing-excludes-auto-scrollbar.enabled undefined true
layout.overflow-underflow.content.enabled undefined false
layout.overflow-underflow.content.enabled_in_addons undefined true
logging.config.modules undefined
media.eme.encrypted-media-encryption-scheme.enabled false true
media.eme.hdcp-policy-check.enabled false true
media.gmp.encoder.enabled undefined false
media.gmp.encoder.multithreaded undefined false
media.gmp.encoder.preferred undefined false
media.navigator.video.disable_h264_baseline undefined false
media.seekToNextFrame.enabled true false
media.webrtc.encoder_creation_strategy undefined 0
media.webrtc.platformencoder false undefined
media.webrtc.platformencoder.sw_only true undefined
media.webrtc.software_encoder.fallback false undefined
memory.phc.avg_delay.content.first undefined 16384
memory.phc.avg_delay.content.normal undefined 4096
memory.phc.avg_delay.content.page_reuse undefined 262144
memory.phc.enabled false true
network.auth.choose_most_secure_challenge true undefined
network.auth.sort_challenge_in_progress undefined false
network.auth.use_redirect_for_retries false true
network.cache.persist_permanent_redirects_http undefined false
network.cookie.CHIPS.enabled undefined false
network.cookie.cookieBehavior.optInPartitioning.pbmode undefined false
network.cookie.sameSite.crossSiteIframeSetCheck undefined true
network.cors_preflight.block_userpass_uri false undefined
network.decompression_off_mainthread true undefined
network.decompression_off_mainthread2 undefined true
network.dns.force_waiting_https_rr true undefined
network.dns.native_https_query false true
network.fetch.redirect.stripAuthHeader true undefined
network.fetchpriority.adjust_urgency undefined true
network.http.http2.enabled.deps true false
network.http.http3.enable_kyber undefined false
network.http.http3.retry_different_ip_family false true
network.http.originextension true undefined
network.http.redirect.stripAuthHeader true undefined
network.jar.record_failure_reason false true
network.predictor.enable-prefetch false true
network.proxy.socks5_remote_dns undefined true
network.send_OnDataFinished.cssLoader false true
network.trr.fetch_off_main_thread true undefined
network.url.strip-data-url-whitespace false undefined
pdfjs.enableHighlightFloatingButton false true
plugins.rewrite_youtube_embeds true false
privacy.annotate_channels.strict_list.enabled false true
privacy.bounceTrackingProtection.enabled false true
privacy.sanitize.clearOnShutdown.hasMigratedToNewPrefs false undefined
privacy.sanitize.clearOnShutdown.hasMigratedToNewPrefs2 undefined false
privacy.sanitize.cpd.hasMigratedToNewPrefs false undefined
privacy.sanitize.cpd.hasMigratedToNewPrefs2 undefined false
privacy.sanitize.useOldClearHistoryDialog true false
reader.font_type.values undefined ["sans-serif","serif","monospace"]
reader.font_weight undefined regular
reader.font_weight.values undefined ["regular","light","bold"]
screenshots.browser.component.preventContentEvents undefined true
security.allow_disjointed_external_uri_loads false undefined
security.disallow_non_local_systemprincipal_in_tests undefined false
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.tls.enable_certificate_compression_brotli undefined false
security.tls.enable_certificate_compression_zlib undefined false
security.webauthn.webauthn_enable_android_fido2.residentkey false true
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
startup.homepage_override_nimbus_minVersion undefined
svg.nearestAndFarthestViewportElement.enabled true false
toolkit.contentRelevancy.log undefined false
toolkit.telemetry.dap_leader https://dap-07-1.api.divviup.org/ https://dap-09-3.api.divviup.org/
toolkit.telemetry.enabled false true
toolkit.telemetry.geckoview.streaming false undefined
toolkit.telemetry.translations.logLevel undefined Error
ui.new-webcompat-reporter.send-more-info-link false true
widget.non-native-theme.solid-outline-style false undefined