Stop recording button broken in Firefox

[mmcc]

In Firefox 92, once I start screensharing in Firefox the "stop recording" button becomes non-functional.

[aminamos]

Tested this in version 96.0.2 and the "stop recording" button worked

[decepulis]

Might still be an issue

User suspects it's because of shared windows and gave us these screenshots!

[philcluff]

Fixed in #221