I can’t find that option in my login screen’s gear icon menu. I think that the reason why I can’t use Wayland is because I have a Radeon HD5450 graphics card that is 15 years old.

I changed it via Debian’s Settings app.

I’m using X11 and Gnome.

I have Flatpak Firefox and it’s giving me the same issue as Firefox ESR.

I’m using Firefox ESR because Flatpak Firefox started freezing after a couple of updates and ESR is super stable.

As for my desktop: Gnome - X11

Lowered my monitor’s refresh rate to 60 Hz and it didn’t resolve the issue

Can you run a benchmark, maybe this one

Can you see something strange in about:processes?

I can’t find anything suspicious but here’s a snippet of it:

Can you try it in other browsers?

I used Ungoogled Chromium and it opens the pop-ups smoothly (so Firefox is causing the problem):

Clip

I disabled recommended performance settings and hardware acceleration in Firefox and I’m still experiencing this issue: