SUMMARY
If I mute and then unmute some youtube video in firefox, it will change the volume that I have configured for the application in the sound panel.

STEPS TO REPRODUCE
1. Install: media player integration, firefox, firefox extension for the integration.
2. Put the built-in sound control widget in your panel.
3. Configure your pulseaudio or wireplumber to have independent volumes per application.
4. Launch firefox, open an incognito window, browse some random video in youtube and play it.
5. While the video is playing, open the sound widget and change the volume for firefox.
6. In the video that is playing in firefox, mute it through the youtube mute button, then unmute it from there.

OBSERVED RESULT
Resets firefox's volume to 100%.

EXPECTED RESULT
Should leave the volume where I left it.

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux
KDE Plasma Version: 5.24.5
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.2
Kernel Version: 5.17.0-2-amd64 (64-bit)
Graphics Platform: X11
Processors: 16 × AMD Ryzen 7 2700 Eight-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: AMD VEGA10