According to [http://www.mozilla.com/en-US/firefox/4.0/system-requirements/ Mozilla Firefox 4 System Requirements], it should be compatible with Windows 2000. But there might not be many beta testers still using 2000, so you might notice more issues than XP/Vista/7 users. Firefox 4 may exercise functions of your graphics card or DirectX that Microsoft has not updated for some time. If you have the energy to track down updated device drivers, that might help. If not, you could turn off hardware acceleration under Tools > Options > Advanced > General sub-tab

Thanks very much! I'll try it out and post the results. . .