Hi again,
As much as I want to, I just wasn't able to recreate the bug you're facing - tried both on my Linux box(arch btw) and on my steam deck - no issue on Firefox whatsoever.
While I can't hold you hostage in debugging this for me, what I did was I uploaded downloadable builds for both Win and Linux. The Linux build I've tested both as just running the executable and adding in steam as a non-steam game. I realize this may put you and other people off(myself included, not really into running random executables from the internet), but that's honestly what I can do for now.
Worth saying: the Deck runs(if I'm not terribly mistaken) the same GPU family and driver stack as your 6500 XT, so not reproducing there is a bit more meaningful than "works on my machine"; it makes me suspect something recent in Mesa rather than your card. You're on 26.1.6, which is well ahead of what SteamOS ships, so might be worth a retry after your next Mesa update.
Also: the Linux download may need a chmod +x before it'll run;
last but not least, even if you don't end up trying the game - many thanks for the provided feedback, it's all been very useful to me
