Mozilla Firefox Optimized for Yahoo! System Requirements
Windows 2000, XP, Server 2003, Vista or 7
Dear Yahoo! do NOT rub salt in my wound.
Looks like Yahoo! haven't updated the System Requirements on their site for a while.
Have you contacted them to notify them of their error?
Note that for now you can still use the Firefox 10 ESR version on Windows 2000 until they switch to Firefox 18.
That version still gets security updates.
I tweeted Yahoo and they said they would look into it.
I've looked into Firefox 10 ESR and I think I'll stick with Firefox 12.
Firefox 10.0.* is more up to date security wise compared to the old Firefox 12.0 as it does not have the security patches that the newer versions have.
Also you may be able to get Firefox 13.0 and newer to work on Win2000 as per http://forums.mozillazine.org/viewtopic.php?f=23&t=2482475
here is how i installed firefox 16 on windows 2000
if this is confusing just reply for help