Firefox came at a time when the web was really annoying to use, and had features to make it better. I don't think IE had even shipped a pop-up blocker at the time. But FireFox's extensibility also made things like ad blockers and Grease Monkey scripts possible, which was the only way to stay sane as a power user in that era. And tabs! That was a big one.

I think the web has gotten annoying again, in different ways, because of the thousand things that nag you on every website to click to close them or otherwise obstruct your reading (cookie banners, too). If a browser can solve that reliably, I'd switch to it.