Are Vimperator + uBlock origin the only active add-ons? The Ubuntu Modifications one is known to cause slowdowns, for example.

What happens if you try to completely reset the Firefox profile, and bring back the plugins you want?

Happens without any extensions. I'm on Arch Linux, not Ubuntu.

I don't care enough to debug it further. Firefox is stable enough these days that restarting it isn't even a daily occurence. I have enough patience to wait that 10+ seconds, it's just a minor annoyance.