Volume Mixer in Windows shows a volume slider for every application that plays audio/video, this applies to web browsers as well, Firefox has a strange bug related to Electrolysis (e10s) and present in current stable Firefox 74 also, where the mixer displays sliders for all open tabs.
As of now, if you launch Firefox and open a few tabs, you’ll notice volume sliders for Firefox gets increased in the volume mixer and don’t vanish when you close Firefox, this problem has been finally fixed by Mozilla in Firefox 76 Nightly version, where currently it shows only one slider irrespective of the number of content processes running.
As we said above, the issue can be reproduced in the Firefox release version. Open the Volume mixer by running sndvol.exe command in the Run dialog or right-click on the volume icon in the notification area and select the option, launch Firefox, open some new tabs to notice more than one volume slider (check the screenshot below).
In the past with Firefox 60 release, Mozilla has addressed a similar issue where new processes were created in volume mixer for each open tab, the issue still persists and the company has resolved the bug by removing Audio Session from content processes on Windows as Mozilla has enabled the necessary Audio IPC on Windows (IPC stands for Inter Processes Communication). Behind the scenes, Mozilla toggles media. pref to true in Firefox Nightly
Related articles:
Firefox Nightly with Media Session API support lets you Control Media with Keyboard Multimedia Keys
Firefox Nightly showing Media Controls on Windows 10 Lock Screen
Firefox 74 lets you control Video Playback in Picture-in-Picture Window with Keyboard
Mozilla Fixes Firefox Audio Playback Issue in Windows 10 build 17063
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