This article applies to Windows only.
If your Windows volume mixer has a slider for each open Firefox tab, this is a bug related to multiprocess (E10s) that will be fixed in Firefox version 76.
In the meantime, in Firefox versions previous to 76, you can set the number of Firefox processes to "1" to resolve the issue:
Setting the number of processes to 1 effectively turns off the multiprocess feature, which may significantly affect Firefox's performance.
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