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

Preference difference
PrefFirefox 50.0.2Firefox 51.0 beta 6
accessibility.typeaheadfind.matchesCountTimeout100undefined
app.update.cert.checkAttributesfalseundefined
browser.migrate.automigrate.enabledfalsetrue
browser.safebrowsing.provider.google4.updateURLhttps://safebrowsing.googleapis.com/v4/threatListUpdates:fetch?$req=%REQUEST_BASE64%&$ct=application/x-protobuf&key=%GOOGLE_API_KEY%https://safebrowsing.googleapis.com/v4/threatListUpdates:fetch?$ct=application/x-protobuf&key=%GOOGLE_API_KEY%
browser.search.reset.enabledfalsetrue
devtools.devedition.promo.enabledfalsetrue
dom.mozAlarms.enabledfalseundefined
dom.vr.add-test-devices0undefined
dom.vr.cardboard.enabledfalseundefined
dom.vr.oculus050.enabledtrueundefined
gfx.content.azure.backendscairoskia
gfx.vr.mirror-texturesfalseundefined
image.single-color-optimization.enabledtrueundefined
javascript.options.mem.gc_per_compartmenttrueundefined
layers.shared-buffer-provider.enabledfalsetrue
layout.css.vertical-text.enabledtrueundefined
media.block-play-until-visiblefalseundefined
media.decoder.heuristic.dormant.timeout6000010000
media.getusermedia.screensharing.allowed_domainswebex.com,*.webex.com,ciscospark.com,*.ciscospark.com,projectsquared.com,*.projectsquared.com,*.room.co,room.co,beta.talky.io,talky.io,*.clearslide.com,appear.in,*.appear.in,tokbox.com,*.tokbox.com,*.sso.francetelecom.fr,*.si.francetelecom.fr,*.sso.infra.ftgroup,*.multimedia-conference.orange-business.com,*.espacecollaboration.orange-business.com,free.gotomeeting.com,g2m.me,*.g2m.me,*.mypurecloud.com,*.mypurecloud.com.au,spreed.me,*.spreed.me,*.spreed.com,air.mozilla.org,*.circuit.com,*.yourcircuit.com,circuit.siemens.com,yourcircuit.siemens.com,circuitsandbox.net,*.unify.com,tandi.circuitsandbox.net,*.ericsson.net,*.cct.ericsson.net,*.opentok.com,*.conf.meetecho.com,meet.jit.si,*.meet.jit.si,web.stage.speakeasyapp.net,web.speakeasyapp.net,*.hipchat.me,*.beta-wspbx.com,*.wspbx.com,*.unifiedcloudit.com,*.smartboxuc.com,*.smartbox-uc.com,*.panterranetworks.com,pexipdemo.com,*.pexipdemo.com,pex.me,*.pex.me,*.rd.pexip.com,1click.io,*.1click.io,*.fuze.com,*.fuzemeeting.com,*.thinkingphones.com,gotomeeting.com,*.gotomeeting.com,gotowebinar.com,*.gotowebinar.com,gototraining.com,*.gototraining.com,citrix.com,*.citrix.com,expertcity.com,*.expertcity.com,citrixonline.com,*.citrixonline.com,g2m.me,*.g2m.me,gotomeet.me,*.gotomeet.me,gotomeet.at,*.gotomeet.at,miriadaxdes.miriadax.net,certificacion.miriadax.net,miriadax.net,*.wire.com,sylaps.com,*.sylaps.com,bluejeans.com,*.bluejeans.com,*.a.bluejeans.comwebex.com,*.webex.com,ciscospark.com,*.ciscospark.com,projectsquared.com,*.projectsquared.com,*.room.co,room.co,beta.talky.io,talky.io,*.clearslide.com,appear.in,*.appear.in,tokbox.com,*.tokbox.com,*.sso.francetelecom.fr,*.si.francetelecom.fr,*.sso.infra.ftgroup,*.multimedia-conference.orange-business.com,*.espacecollaboration.orange-business.com,free.gotomeeting.com,g2m.me,*.g2m.me,*.mypurecloud.com,*.mypurecloud.com.au,spreed.me,*.spreed.me,*.spreed.com,air.mozilla.org,*.circuit.com,*.yourcircuit.com,circuit.siemens.com,yourcircuit.siemens.com,circuitsandbox.net,*.unify.com,tandi.circuitsandbox.net,*.ericsson.net,*.cct.ericsson.net,*.opentok.com,*.conf.meetecho.com,meet.jit.si,*.meet.jit.si,web.stage.speakeasyapp.net,web.speakeasyapp.net,*.hipchat.me,*.beta-wspbx.com,*.wspbx.com,*.unifiedcloudit.com,*.smartboxuc.com,*.smartbox-uc.com,*.panterranetworks.com,pexipdemo.com,*.pexipdemo.com,pex.me,*.pex.me,*.rd.pexip.com,1click.io,*.1click.io,*.fuze.com,*.fuzemeeting.com,*.thinkingphones.com,gotomeeting.com,*.gotomeeting.com,gotowebinar.com,*.gotowebinar.com,gototraining.com,*.gototraining.com,citrix.com,*.citrix.com,expertcity.com,*.expertcity.com,citrixonline.com,*.citrixonline.com,g2m.me,*.g2m.me,gotomeet.me,*.gotomeet.me,gotomeet.at,*.gotomeet.at,miriadaxdes.miriadax.net,certificacion.miriadax.net,miriadax.net,*.wire.com,sylaps.com,*.sylaps.com,bluejeans.com,*.bluejeans.com,*.a.bluejeans.com,*.bbcollab.com
network.http.spdy.enabled.v3-1falseundefined
security.insecure_password.ui.enabledfalsetrue
security.pki.sha1_enforcement_level34
signon.rememberSignons.visibilityTogglefalsetrue
signon.ui.experimentalfalseundefined
startup.homepage_welcome_urlhttps://www.mozilla.org/%LOCALE%/firefox/%VERSION%/firstrun/
webgl.enable-webgl2falsetrue
Default pref Javascript files
PrefFirefox 50.0.2Firefox 51.0 beta 6
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