This is difference between the default preference of Firefox 52.0 beta 3 and one of 51.0.1.

Preference difference
PrefFirefox 51.0.1Firefox 52.0 beta 3
app.update.cert.requireBuiltInfalseundefined
app.update.timerFirstIntervalundefined30000
bidi.support1undefined
browser.crashReports.unsubmittedCheck.enabledfalsetrue
browser.search.geoip.timeout20003000
browser.search.redirectWindowsSearchfalseundefined
browser.shell.skipDefaultBrowserCheckOnFirstRuntruefalse
browser.tabs.crashReporting.requestEmailundefinedfalse
browser.tabs.delayHidingAudioPlayingIconMSundefined3000
devtools.apps.forbidden-permissionsembed-apps,embed-widgetsembed-apps
devtools.command-button-pick.enabledtrueundefined
devtools.debugger.client-source-maps-enabledundefinedtrue
devtools.debugger.promisefalseundefined
devtools.devedition.promo.enabledfalsetrue
devtools.layoutview.enabledundefinedfalse
devtools.responsive.html.enabledfalsetrue
devtools.webconsole.filter.debugundefinedtrue
devtools.webconsole.filter.netundefinedfalse
devtools.webconsole.ui.filterbarundefinedfalse
dom.MediaError.message.enabledfalseundefined
dom.apps.reset-permissionsfalseundefined
dom.cellbroadcast.enabledfalseundefined
dom.forms.datetime.timepickerundefinedfalse
dom.gamepad.extensions.enabledundefinedfalse
dom.icc.enabledfalseundefined
dom.idle_period.throttled_lengthundefined10000
dom.image.picture.enabledtrueundefined
dom.image.srcset.enabledtrueundefined
dom.manifest.onappinstalledundefinedfalse
dom.manifest.oninstallfalseundefined
dom.mobileconnection.enabledfalseundefined
dom.mozApps.debugfalseundefined
dom.mozApps.maxLocalId1000undefined
dom.mozApps.signed_apps_installable_fromhttps://marketplace.firefox.comundefined
dom.requestIdleCallback.enabledundefinedfalse
dom.select_events.enabledfalsetrue
dom.select_events.textcontrols.enabledundefinedfalse
dom.sms.defaultServiceId0undefined
dom.sms.enabledfalseundefined
dom.sms.maxReadAheadEntries0undefined
dom.sms.requestStatusReporttrueundefined
dom.sms.strict7BitEncodingfalseundefined
dom.telephony.defaultServiceId0undefined
dom.telephony.enabledfalseundefined
dom.undo_manager.enabledfalseundefined
dom.voicemail.defaultServiceId0undefined
dom.voicemail.enabledfalseundefined
dom.w3c_pointer_events.implicit_captureundefinedfalse
dom.webnotifications.requireinteraction.countundefined3
dom.webnotifications.requireinteraction.enabledundefinedfalse
gfx.canvas.skiagl.dynamic-cacheundefinedtrue
identity.fxaccounts.contextParamundefinedfx_desktop_v3
idle_queue.long_periodundefined50
idle_queue.min_periodundefined3
javascript.options.shared_memoryfalsetrue
layers.draw-mask-debugundefinedfalse
layout.accessiblecaret.enabled_on_touchfalsetrue
layout.accessiblecaret.hide_carets_for_mouse_inputundefinedtrue
layout.accessiblecaret.use_long_tap_injectortruefalse
layout.css.grid.enabledfalsetrue
layout.css.masking.enabledtrueundefined
layout.css.touch_action.enabledfalsetrue
layout.idle_period.required_quiescent_framesundefined2
layout.idle_period.time_limitundefined1
logging.config.clear_on_startupundefinedtrue
media.decoder-doctor.notifications-allowedMediaWMFNeeded,MediaWidevineNoWMFNoSilverlight,MediaUnsupportedLibavcodecMediaWMFNeeded,MediaWidevineNoWMFNoSilverlight,MediaCannotInitializePulseAudio,MediaCannotPlayNoDecoders,MediaUnsupportedLibavcodec
media.decoder.heuristic.dormant.enabledtrueundefined
media.decoder.heuristic.dormant.timeout10000undefined
media.decoder.recycle.enabledundefinedfalse
media.dormant-on-pause-timeout-msundefined-1
media.format-reader.oggtrueundefined
media.navigator.load_adapt.encoder_onlyundefinedtrue
media.ondevicechange.enabledfalsetrue
media.peerconnection.dtmf.enabledundefinedtrue
media.wave.decoder.enabledtrueundefined
narrate.filter-voicesundefinedtrue
narrate.voiceautomatic { "default": "automatic" }
network.auth.private-browsing-ssoundefinedfalse
network.captive-portal-service.enabledfalsetrue
network.cookie.leave-secure-aloneundefinedtrue
network.http.enable-packaged-appsfalseundefined
network.http.max_response_header_sizeundefined393216
network.http.referer.XOriginTrimmingPolicyundefined0
network.http.sendSecureXSiteReferrertrueundefined
network.http.signed-packages.enabledfalseundefined
network.http.spdy.default-hpack-bufferundefined65536
network.predictor.enable-prefetchfalsetrue
network.tcp.keepalive.probe_countundefined4
prompts.authentication_dialog_abuse_limitundefined3
security.apps.privileged.CSP.defaultdefault-src * data: blob:; script-src 'self'; object-src 'none'; style-src 'self' 'unsafe-inline'undefined
security.csp.enableStrictDynamicundefinedtrue
security.enterprise_roots.enabledundefinedfalse
security.insecure_field_warning.contextual.enabledundefinedtrue
security.mixed_content.hsts_priming_cache_timeoutundefined10080
security.mixed_content.send_hsts_primingtruefalse
security.pki.certificate_transparency.modeundefined1
security.tls.version.max34
services.sync.log.logger.engine.extension-storageundefinedDebug
services.sync.registerEnginesBookmarks,Form,History,Password,Prefs,Tab,AddonsBookmarks,Form,History,Password,Prefs,Tab,Addons,ExtensionStorage
services.sync.sendTabToDevice.enabledfalsetrue
services.sync.validation.intervalundefined86400
services.sync.validation.maxRecordsundefined100
services.sync.validation.percentageChanceundefined10
signon.autofillForms.httpundefinedfalse
webextensions.storage.sync.enabledundefinedfalse
webextensions.storage.sync.serverURLundefinedhttps://webextensions.settings.services.mozilla.com/v1
Default pref Javascript files
PrefFirefox 51.0.1Firefox 52.0 beta 3
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/services-sync.jspresentpresent
omni.ja.d/greprefs.jspresentpresent