User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0

Steps to reproduce:

1. Open a new tab and mute it, or just mute any tab.
2. After muting the tab, access sites that play sounds through contents that use certain plugins(?) through it. I'm not sure exactly what sites have this issue, but this one example (game) should play sound even when the tab should be muted:
   https://www.kongregate.com/games/SoulGame/swords-and-souls

Actual results:

My muted Firefox tab was playing sounds.

Expected results:

My muted Firefox tab should be muted.

Note: I posted this originally a few years ago, but I didn't know how to reproduce or the source. I might have a lead now.
I of course cannot screenshot a sound glitch, else I would. I can record it, if it really matters. I'm also confident the issue is not on my end, but an actual bug on Firefox.

Hi reporter and thank you for taking the time to report this issue.
However, unfortunately, it is an older known issue that still has not been fixed.
I'll close this as a duplicate of bug 1354992, where you can follow the evolution of the issue.