This is difference between the default preference of Firefox 76.0 beta 3 and one of 75.0. This diff table is generated with my small script firefox-prefjs-surveyer.

Preference difference
PrefFirefox 75.0Firefox 76.0 beta 3
app.normandy.onsync_skew_sec 3300 600
app.normandy.test.control1 true undefined
app.releaseNotesURL https://www.mozilla.org/%LOCALE%/firefox/%VERSION%/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnew https://www.mozilla.org/%LOCALE%/firefox/%VERSION%beta/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnew
app.update.channel release beta
app.update.url https://aus5.mozilla.org/update/6/%PRODUCT%/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%SYSTEM_CAPABILITIES%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml 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/firefox/ https://www.mozilla.org/firefox/beta
apz.zoom-to-focused-input.enabled undefined true
browser.aboutwelcome.enabled false true
browser.aboutwelcome.temp.testExperiment.branch undefined control
browser.aboutwelcome.temp.testExperiment.slug undefined
browser.cache.offline.storage.enable true false
browser.display.document_color_use 0 1
browser.in-content.dark-mode true undefined
browser.newtabpage.activity-stream.telemetry.ut.events false true
browser.osKeyStore.loglevel undefined Warn
browser.safebrowsing.provider.google.reportMalwareMistakeURL https://%LOCALE%.malware-error.mozilla.com/?hl=%LOCALE%&url= https://%LOCALE%.malware-error.mozilla.com/?url=
browser.safebrowsing.provider.google.reportPhishMistakeURL https://%LOCALE%.phish-error.mozilla.com/?hl=%LOCALE%&url= https://%LOCALE%.phish-error.mozilla.com/?url=
browser.safebrowsing.provider.google.reportURL https://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&hl=%LOCALE%&site= https://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&site=
browser.safebrowsing.provider.google4.reportMalwareMistakeURL https://%LOCALE%.malware-error.mozilla.com/?hl=%LOCALE%&url= https://%LOCALE%.malware-error.mozilla.com/?url=
browser.safebrowsing.provider.google4.reportPhishMistakeURL https://%LOCALE%.phish-error.mozilla.com/?hl=%LOCALE%&url= https://%LOCALE%.phish-error.mozilla.com/?url=
browser.safebrowsing.provider.google4.reportURL https://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&hl=%LOCALE%&site= https://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&site=
browser.safebrowsing.reportPhishURL https://%LOCALE%.phish-report.mozilla.com/?hl=%LOCALE%&url= https://%LOCALE%.phish-report.mozilla.com/?url=
browser.ssb.enabled undefined false
browser.tabs.remote.allowLinkedWebInFileUriProcess true false
browser.tabs.remote.useCORP true undefined
devtools.performance.popup.feature-flag undefined false
devtools.performance.recording.duration.remote undefined 0
devtools.performance.recording.entries.remote undefined 10000000
devtools.performance.recording.features ["js","leaf","stackwalk"] ["js","leaf","stackwalk","screenshots"]
devtools.performance.recording.features.remote undefined ["js","leaf","stackwalk","screenshots","java"]
devtools.performance.recording.interval.remote undefined 1000
devtools.performance.recording.objdirs.remote undefined []
devtools.performance.recording.preset.remote undefined web-developer
devtools.performance.recording.threads.remote undefined ["GeckoMain","Compositor","Renderer"]
devtools.responsive.touchGestureSimulation.enabled undefined false
dom.IntersectionObserverExplicitDocumentRoot.enabled false true
dom.animations.mainthread-synchronization-with-geometric-animations undefined true
dom.arena_allocator.enabled undefined false
dom.document.exec_command.nested_calls_allowed true false
dom.events.compress.touchmove undefined true
dom.manifest.onappinstalled false undefined
dom.no_unknown_protocol_error.enabled undefined true
dom.registerContentHandler.enabled false undefined
dom.security.https_only_mode undefined false
dom.security.secFetch.enabled undefined false
dom.storage.next_gen false true
dom.targetBlankNoOpener.enabled false true
dom.workers.serialized-sab-access undefined false
extensions.blocklist.interval 86400 undefined
extensions.blocklist.url https://blocklists.settings.services.mozilla.com/v1/blocklist/3/%APP_ID%/%APP_VERSION%/%PRODUCT%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/%PING_COUNT%/%TOTAL_PING_COUNT%/%DAYS_SINCE_LAST_PING%/ undefined
extensions.blocklist.useXML false undefined
extensions.webcompat-reporter.enabled false true
geo.provider.network.url https://www.googleapis.com/geolocation/v1/geolocate?key=%GOOGLE_LOCATION_SERVICE_API_KEY% https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%
gfx.webrender.compositor.force-enabled undefined false
gfx.webrender.split-render-roots false undefined
javascript.options.experimental.await_fix true undefined
javascript.options.experimental.fields true undefined
javascript.options.shared_memory false true
javascript.options.strict false undefined
layout.css.moz-document.url-prefix-hack.enabled true false
layout.dynamic-reflow-roots.enabled false true
media.navigator.video.use_transport_cc undefined false
media.videocontrols.picture-in-picture.video-toggle.flyout-enabled false undefined
media.videocontrols.picture-in-picture.video-toggle.flyout-wait-ms 5000 undefined
media.videocontrols.picture-in-picture.video-toggle.min-video-secs undefined 45
network.cookie.rejectForeignWithExceptions.enabled undefined false
network.cookie.sameSite.laxByDefault.disabledHosts undefined
network.http.http3.recvBufferSize undefined 1048576
network.predictor.cleaned-up false undefined
pdfjs.eventBusDispatchToDOM false undefined
permissions.isolateBy.privateBrowsing false true
privacy.annotate_channels.strict_list.enabled false true
privacy.purge_trackers.logging.level undefined Warn
security.block_Worker_with_wrong_mime false true
security.disallow_non_local_systemprincipal_in_tests undefined false
security.insecure_password.ui.enabled true undefined
security.remote_settings.crlite_filters.enabled false true
security.sandbox.socket.process.level undefined 1
security.tls.version.min 1 3
services.sync.engine.bookmarks.validation.enabled undefined true
services.sync.engine.passwords.validation.enabled undefined true
signon.generation.confidenceThreshold undefined 0.75
signon.management.page.os-auth.enabled undefined true
signon.management.page.vulnerable-passwords.enabled undefined true
signon.passwordEditCapture.enabled false true
toolkit.telemetry.enabled false true
trailhead.firstrun.branches join-dynamic
widget.wayland-cache-mode 0 undefined
widget.wayland-dmabuf-basic-compositor.enabled false undefined
widget.wayland-dmabuf-textures.enabled false undefined
widget.wayland-smooth-rendering undefined false