When you enter a video call in Firefox, it makes this extra mini-toolbar appear. It just has a Firefox logo, a camera icon, and microphone icon ... all of which just do things you can already do using the main toolbar inside Firefox. I imagine the intent is to provide off-screen UI for video calls ... but personally I have no need for this UI: to me it covers up a small portion of other UI I actually need.
But what's worse, as per https://support.mozilla.org/en-US/questions/1209895 there appears to be no official way to remove it (just a CSS hack)!
As a dev myself, I'm sure someone put a lot of work into this mini toolbar ... which is to say someone put a lot of work into making Firefox *worse* for users like me (at least relative to Chrome, which has no such UI) 😞 Ironically, I imagine their intent was the exact opposite: to try to make *better* video call UI.
One checkbox setting ("Display Video Call Mini Toolbar") is all it would take to solve this issue.