Merge m-c to inbound, a=merge

browser/base/content/tabbrowser.xml

...tent/test/popupNotifications/browser_popupNotification.js

browser/components/customizableui/CustomizeMode.jsm

browser/components/uitour/test/browser_UITour.js

browser/modules/test/browser/browser.ini

browser/modules/test/browser/browser_UsageTelemetry.js

...er/modules/test/browser/browser_UsageTelemetry_domains.js

browser/modules/test/browser/head.js

browser/themes/shared/customizableui/panelUI.inc.css

devtools/client/debugger/test/mochitest/browser2.ini

devtools/client/netmonitor/webpack.config.js

dom/media/platforms/agnostic/AOMDecoder.cpp

dom/media/platforms/agnostic/OpusDecoder.cpp

dom/media/platforms/agnostic/TheoraDecoder.cpp

dom/media/platforms/agnostic/VorbisDecoder.cpp

dom/media/platforms/apple/AppleATDecoder.cpp

dom/media/platforms/apple/AppleVTDecoder.cpp

dom/media/platforms/apple/AppleVTDecoder.h

dom/media/platforms/ffmpeg/FFmpegAudioDecoder.cpp

dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp