recently i noticed an issue with my firefox freezing when highlighting text (though i thought it was when i was copying it). luckily its and easy fix if you're using arch linux or an arch based distro

open your terminal and enter this command to install a patched version of the libx11 package paru -S libx11-firefox-fix (replace paru with your aur helper) then just reload your x11 session which you can achieve generally by just logging out

this issue is being tracked here and i recommended checking up on the thread