This doesn't happen for me... and I'm only guessing here....
Some ideas... first update firefox to latest in official repo, which is 94.0 But I mean that isnt the real problem anyway, 92 worked just fine, but can't hurt just to be sure.
sudo apt update && sudo apt upgrade firefox
What is hueman theme? I know firefox has its own themes, did you install that as your firefox theme? Or have you installed that as gtk theme? or what is it? In any case, if you change to another theme, do you still have this issue?