Fantastic. Simply does what it needs to do. I would add an option to make the "muted" tab icon transparent though, it attracts too much attention.

Edit: This is something of a bug in Firefox itself. If I launch a tab with Flash, but don't "allow" the flash to run, and then mute the tab manually via Firefox and THEN click the flash element and allow it to run, the audio will still play and not be muted. I'm not sure it's possible for this add-on to avoid this problem since it is only possible to mute the tab correctly AFTER the flash has been allowed to run.

I'm having the same problem as an earlier reviewer where new tabs with FLASH components are not muted at first. The tab will show the muted symbol, but sound will still play. If I unmute and then mute again, then the tab will actually be muted. Normal tabs which use HTML5 for media ARE automatically muted properly.

Make sure to go to settings and check the for normal window box which is disabled by default which is really dumb if you ask me.