Firefox works fine for me. I had no bugs, did also not notice a system slowdown when using multiple tabs (although working a lot on a G3 Powerbook). It is not fast, but also not so slow that you can't work comfortably and it shows all pages correct, so I use it when safari won't do this.

My problem with it lies at another place: it just does not behave like a mac application. For Example: The search page function is intended to be very comfortable, but if you ever used the basic search function of, let's say safari, you may - like me - prefer that by far: It will open where your eyes are, not at the bottom of page, it will not scroll your page nore beep while you're typing. It will just let you type (and correct your typos) and waits calmly till you hit the bottom to search. Then (with your full attention) it scrolls the page. This is what I want: If I say search, please search, till then wait - it's so easy. This is only an Example - many parts are blown up more and more, want to be sophisticated and are surely not getting better.
But even worse: I am a heavy user and for that do many things with the keyboard. And there are mac-basics that applications should respect in my eyes. For example: When striking key down in a single line of text, the cursor jumps to the beginning. Key up brings it to the Front. I know of other possibilities of course, but that's not the point. I don't like that Firefox ignores this mac-basic in its address bar as well as in the search bar fields, although it does respect it in the HTML-Text-fields. So I stick to safari that tries to behave like the rest of the mac. This continuity makes it possible to work confidently, even blindly and that gives speed.

Nevertheless, I can't rate Firefox bad for that.