This issue reproduces also on https://freeconferencing.vonage.com/ on Mac 10.15 and Win 10, using Firefox Release 84 an Firefox Nightly with the steps provided in comment 5 (the mic doesn't go back again only after refreshing the page).
Bug 1602722 Comment 11 Edit History
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This issue reproduces also on https://freeconferencing.vonage.com/ on Mac 10.15 and Win 10, using Firefox Release 84 an Firefox Nightly 86.0a1 with the steps provided in comment 5 (the mic doesn't go back again only after refreshing the page).