This is difference between the default preference of Firefox 56.0 beta 3 and one of 55.0.2.

Preference difference
PrefFirefox 55.0.2Firefox 56.0 beta 3
app.productInfo.baseURLundefinedhttps://www.mozilla.org/firefox/features/
apz.autoscroll.enabledundefinedfalse
apz.frame_delay.enabledundefinedfalse
apz.highlight_checkerboarded_areasfalseundefined
apz.keyboard.enabledundefinedfalse
apz.one_touch_pinch.enabledfalsetrue
apz.popups.enabledundefinedfalse
browser.crashReports.unsubmittedCheck.enabledfalsetrue
browser.onboarding.hiddenundefinedfalse
browser.onboarding.newtourundefinedprivate,addons,customize,search,default,sync
browser.onboarding.notification.finishedundefinedfalse
browser.onboarding.notification.max-life-time-per-tour-msundefined432000000
browser.onboarding.notification.max-prompt-count-per-tourundefined8
browser.onboarding.notification.mute-duration-on-first-session-msundefined300000
browser.onboarding.tourset-versionundefined1
browser.onboarding.updatetourundefined
browser.places.useAsyncTransactionsundefinedfalse
browser.preferences.searchfalsetrue
browser.preferences.useOldOrganizationtruefalse
browser.safebrowsing.provider.google.advisoryNameundefinedGoogle Safe Browsing.
browser.safebrowsing.provider.google.advisoryURLundefinedhttps://developers.google.com/safe-browsing/v4/advisory
browser.safebrowsing.provider.google4.advisoryNameundefinedGoogle Safe Browsing.
browser.safebrowsing.provider.google4.advisoryURLundefinedhttps://developers.google.com/safe-browsing/v4/advisory
browser.safebrowsing.provider.google4.gethashURLhttps://safebrowsing.googleapis.com/v4/fullHashes:find?$ct=application/x-protobuf&key=%GOOGLE_API_KEY%&$httpMethod=POST
browser.safebrowsing.provider.mozilla.lists.content.descriptionmozfullDescmozfullDesc2
browser.safebrowsing.temporary.take_v4_completion_resultfalseundefined
browser.slowStartup.timeThreshold4000030000
browser.stopReloadAnimation.enabledundefinedtrue
browser.tabs.restorebuttonundefinedfalse
browser.touchmode.autoundefinedfalse
browser.urlbar.match.title#undefined
browser.urlbar.match.url@undefined
browser.urlbar.restrict.bookmark*undefined
browser.urlbar.restrict.history^undefined
browser.urlbar.restrict.openpage%undefined
browser.urlbar.restrict.searches$undefined
browser.urlbar.restrict.tag+undefined
browser.urlbar.restrict.typed~undefined
browser.urlbar.speculativeConnect.enabledundefinedtrue
devtools.debugger.file-search-case-sensitivetruefalse
devtools.debugger.new-debugger-frontendfalsetrue
devtools.debugger.pending-breakpoints[]{}
devtools.devedition.promo.enabledfalsetrue
devtools.gridinspector.gridOutlineMaxColumnsundefined50
devtools.gridinspector.gridOutlineMaxRowsundefined50
devtools.gridinspector.showGridOutlinefalseundefined
devtools.gridinspector.showNegativeLineNumbersundefinedfalse
devtools.inspector.shapesHighlighter.enabledundefinedfalse
devtools.layoutview.enabledfalseundefined
devtools.netmonitor.hiddenColumns["cookies","duration","endTime","latency","protocol","remoteip","responseTime","scheme","setCookies","startTime"]undefined
devtools.netmonitor.visibleColumnsundefined["status","method","file","domain","cause","type","transferred","contentSize","waterfall"]
devtools.promote.layoutviewundefined1
devtools.promote.layoutview.showPromoteBarundefinedtrue
dom.disable_window_showModalDialogundefinedtrue
dom.forms.autocomplete.experimentalfalseundefined
dom.forms.autocomplete.formautofillundefinedfalse
dom.gamepad.haptic_feedback.enabledundefinedtrue
dom.ipc.plugins.asyncInit.enabledfalseundefined
dom.noopener.newprocess.enabledundefinedtrue
dom.performance.time_to_non_blank_paint.enabledundefinedfalse
dom.timeout.background_budget_regeneration_rateundefined100
dom.timeout.background_throttling_max_budgetundefined50
dom.timeout.budget_throttling_max_delayundefined15000
dom.timeout.enable_budget_timer_throttlingundefinedfalse
dom.timeout.foreground_budget_regeneration_rateundefined1
dom.timeout.foreground_throttling_max_budgetundefined-1
dom.timeout.throttling_delayundefined30000
dom.timeout.tracking_throttling_delay30000undefined
editor.use_div_for_default_newlinesfalsetrue
extensions.formautofill.availableundefineddetect
extensions.formautofill.creditCards.enabledundefinedtrue
extensions.formautofill.experimentalfalseundefined
extensions.formautofill.firstTimeUseundefinedtrue
extensions.webextensions.protocol.remoteundefinedtrue
geo.provider.use_gpsdfalseundefined
geo.wifi.urihttps://www.googleapis.com/geolocation/v1/geolocate?key=%GOOGLE_API_KEY%https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%
gfx.webrender.highlight-painted-layersundefinedfalse
gfx.webrender.layers-freeundefinedfalse
gl.use-tls-is-currentundefined0
image.layout_network_priorityundefinedtrue
intl.regional_prefs.use_os_localesundefinedfalse
javascript.options.mem.nursery.max_kbundefined-1
javascript.options.streamsundefinedfalse
layers.popups.compositing.enabledfalseundefined
media.block-autoplay-until-in-foregroundfalsetrue
media.cubeb.log_levelundefined
media.cubeb.logging_levelundefined
media.decoder-doctor.decode-errors-allowedNS_ERROR_DOM_MEDIA_DEMUXER_ERR, NS_ERROR_DOM_MEDIA_METADATA_ERR
media.decoder-doctor.decode-warnings-allowedNS_ERROR_DOM_MEDIA_DEMUXER_ERR, NS_ERROR_DOM_MEDIA_METADATA_ERR
media.decoder.skip-to-next-key-frame.enabledundefinedtrue
media.eme.chromium-api.video-shmems46
media.getusermedia.channelsundefined0
media.memory_cache_max_sizeundefined8192
media.memory_caches_combined_limit_kbundefined524288
media.memory_caches_combined_limit_pc_sysmemundefined5
media.navigator.mediadatadecoder_enabledundefinedfalse
media.peerconnection.video.denoisingundefinedfalse
media.resume-bkgnd-video-on-tabhoverundefinedtrue
media.seekToNextFrame.enabledfalsetrue
network.http.active_tab_priorityundefinedtrue
network.http.on_click_priorityundefinedtrue
network.http.originextensionfalsetrue
network.http.rcwn.max_wait_before_racing_msundefined500
network.http.throttle.enablefalsetrue
network.http.throttle.time-windowundefined3000
network.preloadundefinedtrue
network.standard-url.punycode-hostundefinedfalse
preferences.allow.omt-writeundefinedfalse
security.data_uri.block_toplevel_data_uri_navigationsundefinedfalse
security.mixed_content.hsts_priming_request_timeout30002000
security.mixed_content.send_hsts_primingfalsetrue
security.mixed_content.use_hstsfalsetrue
security.sandbox.content.read_path_whitelistundefined
security.tls.version.max34
services.sync.engine.addressesundefinedfalse
services.sync.engine.addresses.availableundefinedfalse
services.sync.engine.bookmarks.validation.enabledundefinedtrue
services.sync.engine.creditcardsundefinedfalse
services.sync.engine.creditcards.availableundefinedfalse
services.sync.log.logger.engine.addressesundefinedDebug
services.sync.log.logger.engine.creditcardsundefinedDebug
services.sync.registerEnginesBookmarks,Form,History,Password,Prefs,Tab,Addons,ExtensionStorageundefined
services.sync.sendTabToDevice.enabledtrueundefined
toolkit.crashreporter.include_context_heapundefinedfalse
toolkit.datacollection.infoURLundefinedhttps://www.mozilla.org/legal/privacy/firefox.html
toolkit.telemetry.shutdownPingSender.enabledFirstSessionundefinedfalse
toolkit.telemetry.updatePing.enabledundefinedtrue
urlclassifier.disallow_completionstest-malware-simple,test-phish-simple,test-unwanted-simple,test-track-simple,test-trackwhite-simple,test-block-simple,test-flashallow-simple,testexcept-flashallow-simple,test-flash-simple,testexcept-flash-simple,test-flashsubdoc-simple,testexcept-flashsubdoc-simple,goog-downloadwhite-digest256,base-track-digest256,mozstd-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,block-flash-digest256,except-flash-digest256,allow-flashallow-digest256,except-flashallow-digest256,block-flashsubdoc-digest256,except-flashsubdoc-digest256,except-flashinfobar-digest256test-malware-simple,test-phish-simple,test-unwanted-simple,test-track-simple,test-trackwhite-simple,test-block-simple,goog-downloadwhite-digest256,base-track-digest256,mozstd-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256,block-flash-digest256,except-flash-digest256,allow-flashallow-digest256,except-flashallow-digest256,block-flashsubdoc-digest256,except-flashsubdoc-digest256,except-flashinfobar-digest256
urlclassifier.downloadAllowTablegoog-downloadwhite-digest256goog-downloadwhite-proto
urlclassifier.downloadBlockTablegoog-badbinurl-shavargoog-badbinurl-proto
urlclassifier.flashAllowExceptTabletestexcept-flashallow-simple,except-flashallow-digest256except-flashallow-digest256
urlclassifier.flashAllowTabletest-flashallow-simple,allow-flashallow-digest256allow-flashallow-digest256
urlclassifier.flashExceptTabletestexcept-flash-simple,except-flash-digest256except-flash-digest256
urlclassifier.flashSubDocExceptTabletestexcept-flashsubdoc-simple,except-flashsubdoc-digest256except-flashsubdoc-digest256
urlclassifier.flashSubDocTabletest-flashsubdoc-simple,block-flashsubdoc-digest256block-flashsubdoc-digest256
urlclassifier.flashTabletest-flash-simple,block-flash-digest256block-flash-digest256
urlclassifier.malwareTablegoog-malware-shavar,goog-unwanted-shavar,test-malware-simple,test-unwanted-simplegoog-malware-proto,goog-unwanted-proto,test-malware-simple,test-unwanted-simple
urlclassifier.phishTablegoog-phish-shavar,test-phish-simplegoog-phish-proto,test-phish-simple
urlclassifier.update.response_timeout_ms1500030000
urlclassifier.update.timeout_ms6000090000
webgl.max-acceptable-fb-status-invals0undefined
webgl.max-perf-warnings0undefined
webgl.perf.max-acceptable-fb-status-invalsundefined0
webgl.perf.max-warningsundefined0
webgl.perf.spew-frame-allocsundefinedtrue
webrender.highlight-painted-layersfalseundefined
widget.window-transforms.disabledundefinedfalse
Default pref Javascript files
PrefFirefox 55.0.2Firefox 56.0 beta 3
browser/omni.ja.d/defaults/preferences/debugger.jspresentpresent
browser/omni.ja.d/defaults/preferences/devtools.jspresentpresent
browser/omni.ja.d/defaults/preferences/firefox-branding.jspresentpresent
browser/omni.ja.d/defaults/preferences/firefox-l10n.jspresentpresent
browser/omni.ja.d/defaults/preferences/firefox.jspresentpresent
browser/omni.ja.d/defaults/preferences/webide-prefs.jspresentpresent
omni.ja.d/defaults/pref/marionette.jspresentpresent
omni.ja.d/defaults/pref/services-sync.jspresentpresent
omni.ja.d/greprefs.jspresentpresent