Mute sound button in Firefox tabs disable in Flash Player version 20.0.0.235

A speaker button to mute sound will be missing on tabs with Flash content, if Flash Player version 20.0.0.235 is installed. An update is needed from Adobe to fix the issue, which only affects sites using Flash. You should still see a speaker button on tabs for sites that use a different media player, such as YouTube's HTML5 Player.

when does it fix?