This is difference between the default preference of Firefox 126.0 beta 8 and one of 125.0.3. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 125.0.3Firefox 126.0 beta 8
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.drag.initial.enabled true undefined
apz.force_disable_desktop_zooming_scrollbars false undefined
apz.mvm.force-enabled true undefined
browser.backup.enabled false true
browser.messaging-system.whatsNewPanel.enabled true undefined
browser.newtabpage.activity-stream.asrouter.providers.whats-new-panel {"id":"whats-new-panel","enabled":false,"type":"remote-settings","collection":"whats-new-panel","updateCycleInMs":3600000} undefined
browser.newtabpage.activity-stream.feeds.wallpaperfeed undefined true
browser.newtabpage.activity-stream.newtabWallpapers.enabled undefined false
browser.newtabpage.activity-stream.newtabWallpapers.wallpaper-dark undefined
browser.newtabpage.activity-stream.newtabWallpapers.wallpaper-light undefined
browser.sessionstore.max_windows_undo 3 5
corroborator.enabled true undefined
devtools.command-button-splitconsole.enabled true undefined
devtools.toolbox.splitconsole.enabled undefined true
devtools.toolbox.splitconsole.open undefined false
devtools.toolbox.splitconsoleEnabled false undefined
dom.block_reload_from_resize_event_handler false undefined
dom.disable_tab_focus_to_root_element false true
dom.element.customstateset.enabled false true
dom.events.textevent.enabled undefined false
dom.fetchKeepalive.enabled undefined false
dom.innerSize.rounded true undefined
dom.innerSize.rounding undefined 2
dom.input.skip_cursor_move_for_same_value_set true undefined
dom.screenwakelock.enabled false true
dom.security.setdocumenturi false true
dom.shadowdom.selection_across_boundary.enabled undefined false
dom.text_fragments.enabled undefined false
dom.webgpu.allow-present-without-readback undefined false
dom.webgpu.swap-chain.external-texture-dx12 false undefined
dom.webnotifications.loglevel undefined Error
extensions.webcompat-reporter.enabled false true
fission.preserve_browsing_contexts true undefined
fission.remoteObjectEmbed true undefined
gfx.canvas.remote.use-draw-image-fast-path undefined true
gfx.remote-texture.recycle.disabled undefined false
javascript.options.main_process_disable_jit undefined false
layout.css.at-scope.enabled undefined false
layout.css.basic-shape-shape.enabled undefined false
layout.css.letter-spacing.model undefined 0
layout.css.motion-path-basic-shapes.enabled true undefined
layout.css.motion-path-coord-box.enabled true undefined
layout.css.motion-path-offset-position.enabled true undefined
layout.css.motion-path-ray.enabled true undefined
layout.css.prefixes.transforms true false
layout.css.prefixes.transitions true false
layout.css.relative-color-syntax.enabled undefined false
layout.css.starting-style-at-rules.enabled undefined false
layout.css.video-dynamic-range.allows-high undefined false
layout.css.zoom.enabled false true
mathml.centered_operators.disabled undefined true
mathml.top_bottom_spacing_for_stretchy_operators.disabled undefined true
media.cubeb.input_drift_factor undefined 1.0
media.cubeb.output_drift_factor undefined 1.0
media.gmp-manager.allowLocalSources undefined true
media.theora.enabled undefined true
media.widevine.hdcp-protection-mask undefined true
network.decompression_off_mainthread undefined true
network.decompression_off_mainthread_min_size undefined 512
network.dns.preferIPv6 undefined false
network.fetch.systemDefaultsToOmittingCredentials undefined true
network.http.accept-encoding.secure gzip, deflate, br gzip, deflate, br, zstd
network.http.digest_auth_cnonce_length undefined 16
network.http.http2.aggressive_coalescing undefined false
network.http.priority_header.enabled undefined true
network.http.speculative-parallel-limit 6 20
network.url.simple_uri_schemes undefined
network.url.some_schemes_bypass_defaultURI_fallback undefined true
pdfjs.enableHighlightEditor false true
print.center_page_on_sheet undefined 2
privacy.sanitize.useOldClearHistoryDialog true false
prompts.contentPromptSubDialog true undefined
prompts.modalType.httpAuth 2 undefined
prompts.tabChromePromptSubDialog true undefined
prompts.windowPromptSubDialog true undefined
reader.color_scheme.values ["light","dark","sepia","auto"] ["auto","light","dark","sepia","contrast","gray"]
reader.colors_menu.enabled undefined false
reader.custom_colors.background undefined
reader.custom_colors.foreground undefined
reader.custom_colors.selection-highlight undefined
reader.custom_colors.unvisited-links undefined
reader.custom_colors.visited-links undefined
sidebar.companion false undefined
sidebar.revamp undefined false
toolkit.contentRelevancy.enabled undefined false
toolkit.telemetry.enabled false true
toolkit.telemetry.user_characteristics_ping.logLevel undefined Warn