This is difference between the default preference of Firefox 49.0 beta 3 and one of 48.0.

Preference difference
PrefFirefox 48.0Firefox 49.0 beta 3
app.update.incompatible.mode0undefined
app.update.mode1undefined
app.update.promptWaitTime172800691200
app.update.showInstalledUIfalseundefined
apz.scale_repaint_delay_msundefined500
browser.bookmarks.showRecentlyBookmarkedundefinedtrue
browser.eme.ui.enabledfalsetrue
browser.safebrowsing.provider.google.gethashURLhttps://safebrowsing.google.com/safebrowsing/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2https://safebrowsing.google.com/safebrowsing/gethash?client=SAFEBROWSING_ID&appver=%MAJOR_VERSION%&pver=2.2
browser.safebrowsing.provider.google.pverundefined2.2
browser.safebrowsing.provider.google.updateURLhttps://safebrowsing.google.com/safebrowsing/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2&key=%GOOGLE_API_KEY%https://safebrowsing.google.com/safebrowsing/downloads?client=SAFEBROWSING_ID&appver=%MAJOR_VERSION%&pver=2.2&key=%GOOGLE_API_KEY%
browser.safebrowsing.provider.mozilla.gethashURLhttps://shavar.services.mozilla.com/gethash?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2https://shavar.services.mozilla.com/gethash?client=SAFEBROWSING_ID&appver=%MAJOR_VERSION%&pver=2.2
browser.safebrowsing.provider.mozilla.pverundefined2.2
browser.safebrowsing.provider.mozilla.updateURLhttps://shavar.services.mozilla.com/downloads?client=SAFEBROWSING_ID&appver=%VERSION%&pver=2.2https://shavar.services.mozilla.com/downloads?client=SAFEBROWSING_ID&appver=%MAJOR_VERSION%&pver=2.2
browser.search.reset.enabledundefinedtrue
browser.search.reset.whitelistundefined
canvas.filters.enabledfalsetrue
canvas.imagebitmap_extensions.enabledundefinedfalse
devtools.apps.forbidden-permissionsembed-apps,engineering-mode,embed-widgetsembed-apps,embed-widgets
devtools.devedition.promo.enabledfalsetrue
devtools.webconsole.fontSize0undefined
devtools.webconsole.new-frontend-enabledundefinedfalse
dom.animations.offscreen-throttlingundefinedtrue
dom.audiochannel.audioCompetingundefinedfalse
dom.datastore.enabledfalseundefined
dom.details_element.enabledfalsetrue
dom.disable_window_open_feature.scrollbarsfalseundefined
dom.enable_performance_observerundefinedfalse
dom.flyweb.enabledundefinedfalse
dom.global_stop_scriptundefinedtrue
dom.inter-app-communication-api.enabledfalseundefined
dom.push.adaptive.enabledfalseundefined
dom.push.adaptive.gap60000undefined
dom.push.adaptive.lastGoodPingInterval180000undefined
dom.push.adaptive.lastGoodPingInterval.mobile180000undefined
dom.push.adaptive.lastGoodPingInterval.wifi180000undefined
dom.push.adaptive.upperLimit1740000undefined
dom.push.pingInterval.default180000undefined
dom.push.pingInterval.mobile180000undefined
dom.push.pingInterval.wifi180000undefined
dom.push.udp.wakeupEnabledfalseundefined
dom.requestSync.enabledfalseundefined
dom.server-events.enabledtrueundefined
dom.vr.osvr.enabledundefinedfalse
extensions.blocklist.detailsURLhttps://www.mozilla.org/%LOCALE%/blocklist/https://www.mozilla.com/%LOCALE%/blocklist/
gfx.font_rendering.graphite.enabledfalsetrue
gfx.vr.osvr.clientKitLibPathundefined
gfx.vr.osvr.clientLibPathundefined
gfx.vr.osvr.commonLibPathundefined
gfx.vr.osvr.utilLibPathundefined
layout.accessiblecaret.extend_selection_for_phone_numberundefinedfalse
layout.accessiblecaret.timeout_ms30000
layout.css.background-clip-text.enabledfalsetrue
layout.css.prefixes.webkitfalsetrue
layout.css.text-emphasis.enabledtrueundefined
media.decoder-doctor.notifications-allowedMediaWidevineNoWMFNoSilverlightMediaWMFNeeded,MediaWidevineNoWMFNoSilverlight
media.decoder-doctor.wmf-disabled-is-failureundefinedfalse
media.default_volumeundefined1.0
media.gmp-widevinecdm.enabledundefinedfalse
media.gmp-widevinecdm.visibleundefinedtrue
media.navigator.video.use_rembundefinedtrue
media.navigator.video.use_tmmbrundefinedfalse
media.seekToNextFrame.enabledundefinedfalse
media.suspend-bkgnd-video.delay-msundefined10000
media.suspend-bkgnd-video.enabledundefinedfalse
media.webspeech.synth.enabledfalsetrue
network.http.keep_empty_response_headers_as_empty_stringundefinedfalse
print.use_simplify_pageundefinedfalse
reader.content_widthundefined3
reader.line_heightundefined4
security.cert_pinning.max_max_age_secondsundefined5184000
security.pki.netscape_step_up_policyundefined1
security.signed_content.CSP.defaultundefinedscript-src 'self'; style-src 'self'
security.ssl3.ecdhe_ecdsa_aes_256_gcm_sha384undefinedtrue
security.ssl3.ecdhe_rsa_aes_256_gcm_sha384undefinedtrue
services.blocklist.addons.checkedundefined0
services.blocklist.addons.collectionundefinedaddons
services.blocklist.bucketundefinedblocklists
services.blocklist.changes.pathundefined/buckets/monitor/collections/changes/records
services.blocklist.gfx.checkedundefined0
services.blocklist.gfx.collectionundefinedgfx
services.blocklist.onecrl.checkedundefined0
services.blocklist.onecrl.collectionundefinedcertificates
services.blocklist.plugins.checkedundefined0
services.blocklist.plugins.collectionundefinedplugins
services.blocklist.signing.enforcedundefinedfalse
services.blocklist.update_enabledundefinedfalse
services.kinto.addons.checked0undefined
services.kinto.addons.collectionaddonsundefined
services.kinto.basehttps://firefox.settings.services.mozilla.com/v1undefined
services.kinto.bucketblocklistsundefined
services.kinto.changes.path/buckets/monitor/collections/changes/recordsundefined
services.kinto.gfx.checked0undefined
services.kinto.gfx.collectiongfxundefined
services.kinto.onecrl.checked0undefined
services.kinto.onecrl.collectioncertificatesundefined
services.kinto.plugins.checked0undefined
services.kinto.plugins.collectionpluginsundefined
services.kinto.update_enabledfalseundefined
services.settings.serverundefinedhttps://firefox.settings.services.mozilla.com/v1
services.sync.prefs.sync.app.update.modetrueundefined
services.sync.prefs.sync.browser.urlbar.suggest.bookmarkundefinedtrue
services.sync.prefs.sync.browser.urlbar.suggest.historyundefinedtrue
services.sync.prefs.sync.browser.urlbar.suggest.history.onlyTypedundefinedtrue
services.sync.prefs.sync.browser.urlbar.suggest.openpageundefinedtrue
services.sync.prefs.sync.browser.urlbar.suggest.searchesundefinedtrue
signon.schemeUpgradesundefinedtrue
Default pref Javascript files
PrefFirefox 48.0Firefox 49.0 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