Game mode tweaks needed for Firefox and also on screen keyboard

I mostly use Game Mode, but I want to set up Firefox. I set it as a non-steam game, and it opens, but for some reason it's not full screen, there's black bars on both sides. I've tried a few suggestions online like the following:

https://www.reddit.com/r/SteamDeck/comments/16eiyi6/make_firefox_work_better_in_game_mode/?utm_source=share&utm_medium=web2x&context=3

But the command line settings didn't make any difference.

Also, I want to know if it's possible to disable the onscreen keyboard, as it's useless on a mini PC attached to a TV and a wireless keyboard. It still comes up when in Game Mode and you click inside a text field.