This is difference between the default preference of Firefox 50.0 beta 3 and one of 49.0.1.

Preference difference
PrefFirefox 49.0.1Firefox 50.0 beta 3
accessibility.typeaheadfind.matchesCountLimit1001000
accessibility.typeaheadfind.matchesCountTimeout250100
apz.content_response_timeout300400
browser.addon-watch.percentage-limit5undefined
browser.download.forbid_open_withundefinedfalse
browser.migrate.automigrate.enabledfalsetrue
browser.safebrowsing.enabledtrueundefined
browser.safebrowsing.forbiddenURIs.enabledfalseundefined
browser.safebrowsing.phishing.enabledundefinedtrue
browser.safebrowsing.provider.google.listsgoog-badbinurl-shavar,goog-downloadwhite-digest256,goog-phish-shavar,goog-malware-shavar,goog-unwanted-shavargoog-badbinurl-shavar,goog-downloadwhite-digest256,goog-phish-shavar,googpub-phish-shavar,goog-malware-shavar,goog-unwanted-shavar
browser.safebrowsing.provider.google4.gethashURLundefinedhttps://safebrowsing.googleapis.com/v4/fullHashes:find?$req=%REQUEST_BASE64%&$ct=application/x-protobuf&key=%GOOGLE_API_KEY%
browser.safebrowsing.provider.google4.listsundefinedgoog-phish-proto,googpub-phish-proto,goog-malware-proto,goog-unwanted-proto
browser.safebrowsing.provider.google4.pverundefined4
browser.safebrowsing.provider.google4.reportURLundefinedhttps://safebrowsing.google.com/safebrowsing/diagnostic?client=%NAME%&hl=%LOCALE%&site=
browser.safebrowsing.provider.google4.updateURLundefinedhttps://safebrowsing.googleapis.com/v4/threatListUpdates:fetch?$req=%REQUEST_BASE64%&$ct=application/x-protobuf&key=%GOOGLE_API_KEY%
browser.safebrowsing.provider.mozilla.listsmozstd-track-digest256,mozstd-trackwhite-digest256,mozfull-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256base-track-digest256,mozstd-trackwhite-digest256,content-track-digest256,mozplugin-block-digest256,mozplugin2-block-digest256
browser.safebrowsing.provider.mozilla.lists.base.descriptionundefinedmozstdDesc
browser.safebrowsing.provider.mozilla.lists.base.nameundefinedmozstdName
browser.safebrowsing.provider.mozilla.lists.content.descriptionundefinedmozfullDesc
browser.safebrowsing.provider.mozilla.lists.content.nameundefinedmozfullName
browser.safebrowsing.provider.mozilla.lists.mozfull.descriptionmozfullDescundefined
browser.safebrowsing.provider.mozilla.lists.mozfull.namemozfullNameundefined
browser.safebrowsing.provider.mozilla.lists.mozstd.descriptionmozstdDescundefined
browser.safebrowsing.provider.mozilla.lists.mozstd.namemozstdNameundefined
devtools.command-button-eyedropper.enabledfalseundefined
devtools.devedition.promo.enabledfalsetrue
devtools.hud.loglimitundefined1000
devtools.memory.enabledfalsetrue
devtools.sourcemap.locations.enabledundefinedfalse
devtools.toolbox.toolbarSpec["splitconsole", "paintflashing toggle","scratchpad","resize toggle","eyedropper","screenshot --fullpage", "rulers", "measure"]["splitconsole", "paintflashing toggle","scratchpad","resize toggle","screenshot --fullpage", "rulers", "measure"]
dom.gamepad.test.enabledundefinedfalse
dom.input.dirpickerundefinedfalse
dom.ipc.cpows.allow-cpows-in-compat-addonsundefined{b9db16a4-6edc-47ec-a1f4-b86292ed211d},firegestures@xuldev.org,{DDC359D1-844A-42a7-9AA1-88A850A938A8},privateTab@infocatcher,mousegesturessuite@lemon_juice.addons.mozilla.org,treestyletab@piro.sakura.ne.jp,cliqz@cliqz.com,{AE93811A-5C9A-4d34-8462-F7B864FC4696},contextsearch2@lwz.addons.mozilla.org,{EF522540-89F5-46b9-B6FE-1829E2B572C6},{677a8f98-fd64-40b0-a883-b8c95d0cbf17},images@wink.su,fx-devtools,toolkit/require,url_advisor@kaspersky.com,{d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d},{dc572301-7619-498c-a57d-39143191b318},dta@downthemall.net,{86095750-AD15-46d8-BF32-C0789F7E6A32},screenwise-prod@google.com,{91aa5abe-9de4-4347-b7b5-322c38dd9271},secureLogin@blueimp.net,ich@maltegoetz.de,come.back.block.image.from@cat-in-136.blogspot.com,{7b1bf0b6-a1b9-42b0-b75d-252036438bdc},s3crypto@data,{1e0fd655-5aea-4b4c-a583-f76ef1e3af9c},akahuku.fx.sp@toshiakisp.github.io,{aff87fa2-a58e-4edd-b852-0a20203c1e17},{1018e4d6-728f-4b20-ad56-37578a4de76b},rehostimage@engy.us,lazarus@interclue.com,{b2e69492-2358-071a-7056-24ad0c3defb1},flashstopper@byo.co.il,{e4a8a97b-f2ed-450b-b12d-ee082ba24781},jid1-f3mYMbCpz2AZYl@jetpack,{8c550e28-88c9-4764-bb52-aa489cf2efcd},{37fa1426-b82d-11db-8314-0800200c9a66},{ac2cfa60-bc96-11e0-962b-0800200c9a66},igetter@presenta.net,killspinners@byo.co.il,abhere2@moztw.org,{fc6339b8-9581-4fc7-b824-dffcb091fcb7},wampi@wink.su,backtrack@byalexv.co.uk,Gladiator_X@mail.ru,{73a6fe31-595d-460b-a920-fcc0f8843232},{46551EC9-40F0-4e47-8E18-8E5CF550CFB8},acewebextension_unlisted@acestream.org,@screen_maker,yasearch@yandex.ru,sp@avast.com,s3google@translator,igetterextension@presenta.net,{C1A2A613-35F1-4FCF-B27F-2840527B6556},screenwise-testing@google.com,helper-sig@savefrom.net,browser-loader,ImageSaver@Merci.chao,proxtube@abz.agency,wrc@avast.com,{9AA46F4F-4DC7-4c06-97AF-5035170634FE},jid1-CikLKKPVkw6ipw@jetpack,artur.dubovoy@gmail.com,nlgfeb@nlgfeb.ext,{A065A84F-95B6-433A-A0C8-4C040B77CE8A},fdm_ffext@freedownloadmanager.org
dom.ipc.cpows.forbid-cpows-in-compat-addonsundefinedtrue
dom.ipc.plugins.asyncdrawing.enabledfalsetrue
dom.manifest.oninstallundefinedfalse
dom.mapped_arraybuffer.enabledfalsetrue
dom.mozKillSwitch.enabledfalseundefined
dom.presentation.discovery.legacy.enabledundefinedfalse
dom.push.loglevelofferror
dom.push.maxRecentMessageIDsPerSubscriptionundefined10
dom.webcomponents.customelements.enabledundefinedfalse
dom.webkitBlink.dirPicker.enabledundefinedtrue
dom.webkitBlink.filesystem.enabledundefinedtrue
dom.workers.maxPerDomain50undefined
findbar.entirewordundefinedfalse
findbar.highlightAllundefinedfalse
findbar.modalHighlightundefinedfalse
font.name-list.serif.x-mathLatin Modern Math, XITS Math, Cambria Math, TeX Gyre Bonum Math, TeX Gyre Pagella Math, TeX Gyre Schola, TeX Gyre Termes Math, STIX Math, Asana Math, STIXGeneral, DejaVu Serif, DejaVu Sans, serifLatin Modern Math, XITS Math, Cambria Math, Libertinus Math, DejaVu Math TeX Gyre, TeX Gyre Bonum Math, TeX Gyre Pagella Math, TeX Gyre Schola, TeX Gyre Termes Math, STIX Math, Asana Math, STIXGeneral, DejaVu Serif, DejaVu Sans, serif
gfx.filter.nearest.force-enabledfalseundefined
gfx.ycbcr.accurate-conversionundefinedfalse
intl.ime.remove_placeholder_character_at_commitundefinedfalse
javascript.options.mem.gc_decommit_threshold_mb32undefined
javascript.options.wasm_baselinejitundefinedfalse
layers.compositor-lru-size0undefined
layers.shared-buffer-provider.enabledundefinedfalse
layers.tiled-drawtarget.enabledfalseundefined
layout.css.initial-letter.enabledundefinedfalse
media.format-reader.oggundefinedtrue
media.navigator.audio.use_fecundefinedtrue
media.navigator.video.red_ulpfec_enabledundefinedfalse
media.webvtt.enabledtrueundefined
network.http.enablePerElementReferrerfalsetrue
network.http.keep_empty_response_headers_as_empty_stringfalsetrue
network.http.spdy.enabled.v3-1truefalse
places.frecency.reloadVisitBonusundefined0
plugins.navigator_hide_disabled_flashundefinedfalse
plugins.update.notifyUserfalseundefined
plugins.update.urlhttps://www.mozilla.org/%LOCALE%/plugincheck/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=plugincheck-updateundefined
pointer-lock-api.prefixed.enabledundefinedfalse
pointer-lock-api.warning.timeoutundefined3000
print.use_simplify_pagefalsetrue
privacy.userContext.ui.enabledundefinedfalse
security.family_safety.modeundefined2
security.insecure_password.ui.enabledfalsetrue
security.onecrl.via.amotruefalse
security.ssl.errorReporting.urlhttps://data.mozilla.com/submit/sslreportshttps://incoming.telemetry.mozilla.org/submit/sslreports/
security.ssl3.ecdhe_ecdsa_rc4_128_shatrueundefined
security.ssl3.ecdhe_rsa_rc4_128_shatrueundefined
security.ssl3.rsa_rc4_128_md5trueundefined
security.ssl3.rsa_rc4_128_shatrueundefined
services.blocklist.signing.enforcedfalsetrue
services.blocklist.update_enabledfalsetrue
services.sync.addons.ignoreRepositoryCheckingfalseundefined
services.sync.engine.tabs.filteredUrls^(about:.*|chrome://weave/.*|wyciwyg:.*|file:.*)$^(about:.*|chrome://weave/.*|wyciwyg:.*|file:.*|blob:.*)$
services.sync.prefs.sync.browser.ctrlTab.previewsundefinedtrue
services.sync.prefs.sync.browser.download.useDownloadDirundefinedtrue
services.sync.prefs.sync.browser.safebrowsing.enabledtrueundefined
services.sync.prefs.sync.browser.safebrowsing.phishing.enabledundefinedtrue
services.sync.sendTabToDevice.enabledundefinedfalse
services.sync.telemetry.maxPayloadCountundefined500
services.sync.telemetry.submissionIntervalundefined43200
startup.homepage_welcome_urlhttps://www.mozilla.org/%LOCALE%/firefox/%VERSION%/firstrun/
startup.homepage_welcome_url.additionalhttps://www.mozilla.org/%LOCALE%/firefox/%VERSION%/firstrun/learnmore/
urlclassifier.disallow_completionstest-malware-simple,test-phish-simple,test-unwanted-simple,test-track-simple,test-trackwhite-simple,test-forbid-simple,goog-downloadwhite-digest256,mozstd-track-digest256,mozstd-trackwhite-digest256,mozfull-track-digest256,test-block-simple,mozplugin-block-digest256,mozplugin2-block-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
urlclassifier.forbiddenTabletest-forbid-simpleundefined
urlclassifier.trackingTabletest-track-simple,mozstd-track-digest256test-track-simple,base-track-digest256
webchannel.allowObject.urlWhitelistundefinedhttps://accounts.firefox.com https://content.cdn.mozilla.net https://input.mozilla.org https://support.mozilla.org https://install.mozilla.org
webextensions.webRequest.requestBodyMaxRawBytesundefined16777216
webgl.disable-wglundefinedfalse
webgl.enable-prototype-webgl2falseundefined
webgl.enable-webgl2undefinedfalse
Default pref Javascript files
PrefFirefox 49.0.1Firefox 50.0 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