Performance absolutely matters. I've dropped firefox like 5 times in the past 10 years because either it's stability with extensions was bad, or it handled tabs so poorly it felt like memory leaks dragging my entire PC down. At the end of the day you have to actually be able to use the browser.
Performance absolutely matters.
And I am trying firefox again and have experienced two crashes. i'm still giving it a chance but I need to stress that the most important thing is that it fulfills it's main purpose, browsing . If my #1 concern was privacy I wouldn't bother with the browsers altogether.
edit: It is worth mentioning I don't think performance has anything to do with privacy, and firefox could absolutely have both. It just hasn't in my experience.