Win98SE on this machine too, and still running Firefox 18.104.22.168 - No problems at all. I'm thinking about upgrading Firefox, but certainly not Windows. I've had enough trouble with XP et al with a couple of my commercial clients' machines.
I only keep 98SE on this one machine because I sometimes need to test software on it, and it's handy to use this system for general web browsing as well. If it wasn't for that need, I wouldn't use Win-Doze at all.