Fullscreen video in Firefox

Hi Jon!

I just made the switch to Arch due to your video and have been setting things up over the past week. I noticed a small issue with Firefox where going fullscreen still leaves the `Polybar` visible.

I wondered whether you ran into this issue as well and, if so, if you managed to solve it. It seems a [fair amount of people](https://duckduckgo.com/?q=firefox+video+xmonad+fullscreen ) mention this issue, yet none of the solutions seem to have worked for me. Perhaps I glossed over something that you are aware of.

Here is my [`xmonad.hs`](https://github.com/imjasonmiller/configs/blob/0003497856704d1996d8ddef8cc71494a569c44e/gui/.xmonad/xmonad.hs) config.

Thanks for sharing the configs and many tips!