As the title says I've noticed that in the past day kBin often freezes up when I view a thread (Firefox pops up asking me if I want to stop the tab or not). I use Firefox on EndeavourOS but I didn't have any problems with it for the past 2 weeks.
Itemize what extensions you are running and try creating a vanilla prefix (firefox -P), then testing it there, and use process of elimination to narrow down which one(s) may be the root cause.
Well, if you aren't running browser extensions (add-ons) in your browser, then that can't be the cause. Sometimes extensions may conflict with the browser and drag performance, so it can be good to get closer to a vanilla state, but sounds like you already are there.
Sorry for misunderstanding, I should've been more clear: I do use extensions, namely uBlock, Tampermonkey and bitwarden, but I was also using them when everything ran smooth.
What I meant I didn't quite understand was "try creating a vanilla prefix (firefox -P), then testing it there".
You can create a new prefix/profile with no extensions in it and use this as a "control" to test whether it works without those. Then, on your normal profile, start by enabling only one extension, then two, etc., to try to drill down to see if it's localized to a particular one.