I'm not sure how this contributes to the thread. This isn't a technical support forum, so it might not be the best place to discuss specific browser issues.

I've been using Firefox on Ubuntu since 18.04 was first released (about 6 years ago), and while I've encountered some issues, I haven't experienced the problem you're describing.

Of course, browser performance can be affected by many factors in your system. If you're seeking help, you might have better luck in a dedicated support forum or the official Firefox support channels.