I'm on a G3 with OS X 10.2.8 and 768 MB Ram. I love everything about Firefox except one, but it's a biggie:

At some point, after I've been using it for a while, it gets caught in a loop and the spinning beach ball of death becomes nearly perpetual, with an occasional short break. I can't close the page, I can't stop it with the Stop icon in the toolbar, and I can't stop it with Command-period, Escape, or any of the old standbyes. I can't quit it, either (the Dock reports Application Not Responding). Eventually, after being frozen from working, like it or not, I just have to force quit it and re-launch it. This is not only a royal pain and time-waster, but worse, I may have had several pages and tabs open, and I lose the thread of what I was researching or doing.

I'd give Firefox five stars for everything else, but this one fault is an incredibly strong offset to otherwise stellar performance and features, as well as ease of use.

I had hoped this latest update 1.0.4 would fix it, but alas, it's still got this issue.