I went looking for log files ([HowTo] Find error logs) and found this SE question + answer.
~/.mozilla/firefox/Crash\ Reports/events/ was empty, but journalctl -qb /usr/bin/firefox included this line a few times:
Dec 06 17:19:25 pancelor-manjaro plasmashell[10455]: [GFX1-]: glxtest: VA-API test failed: process crashed. Please check your VA-API drivers.
I found a reddit post which led me to a mozilla bug and an x11 bug. This seems like my issue! I can now consistently reproduce the freeze by having a video running in one firefox window and highlighting text in another firefox window.
A reddit commentor says “Temporary Solution: Downgrading to libx11 1.8.1-3 solves this issue”.
After some more tutorial searching, I’m looking at my cached libx11 packages:
ls /var/cache/pacman/pkg/libx11*
/var/cache/pacman/pkg/libx11-1.8.1-2-x86_64.pkg.tar.zst /var/cache/pacman/pkg/libx11-1.8.2-1-x86_64.pkg.tar.zst /var/cache/pacman/pkg/libx11-1.8.2-2-x86_64.pkg.tar.zst
/var/cache/pacman/pkg/libx11-1.8-1-x86_64.pkg.tar.zst /var/cache/pacman/pkg/libx11-1.8.2-1-x86_64.pkg.tar.zst.sig /var/cache/pacman/pkg/libx11-1.8.2-2-x86_64.pkg.tar.zst.sig
Cool, I guess I’ll downgrade to 1.8.1-2. (sudo pacman -U /var/cache/pacman/pkg/libx11-1.8.1-2-x86_64.pkg.tar.zst)
Will this break anything in some way I’m not expecting? Will my libx11 get updated next release, or do I need to remember to manually un-downgrade it?
I suppose I’ve mostly answered my question at this point, but at the very least this thread might be useful if others have the same issue. I’ve cobbled together a solution (I haven’t tested it yet) but if any experts out there have holistic advice/comments I’d appreciate it. (or specific comments – maybe my solution doesn’t even work! I’ll try it in a few days if this keeps being an issue)