What version works best with Windows2000

I have a PC with 2 GBs of RAM, running W2K, primarily. I had avoided upgrading from the Firefox 3.x.x series ( or 5 ? ) because it was my understanding that it was the last one suitable for Windows2000. Not long ago, I got an automatic robot request from Firefox to update, and tried to make sure it was still within the compatibility range for W2k. I thought that I was getting a minor update to a W2K version, but instead, have Firefox 12, which has totally disrupted my internet activity because it constantly bogs down into barely running, or else freezes up for long periods, and frequently crashes. I want to roll back to whatever I'd used before this disaster. And if there's any special ways to do that, I want to know how it works. Thanks. Kiwi