You will not be able to run Firefox 4, the system requirements include hardware recommendations of Pentium 4 or newer processor and at least 512MB of RAM.

http://www.mozilla.com/en-US/firefox/4.0/system-requirements/

I'm running FF4 RC on Windows 2000, 384 MB RAM. So it works. In fact, it works very well! So, well done Mozilla/Firefox team!

The Firefox 4.0 RC2 version has just been released, so you can update via Help > About ...

Actually, Firefox 4 WILL run under Windows 2000 with 128MB RAM.

If you had carefully read the system requirements on the link you provided,

http://www.mozilla.com/en-US/firefox/4.0/system-requirements/

you would have noticed it says "Recommended Hardware." That is quite different from "Minimum Hardware" or "Minimum Requirements."

This is why "jackfeed" was able to run it on 384MB RAM.

Be aware that if using a processor and RAM that are below recommended requirements you are likely to have performance problems.