As far as I know, Firefox is not "optimized" for particular web sites. If you see a version of Firefox that claims to be "optimized for Yahoo," it might be a new flavor created by Yahoo, but that's just a guess.

Keep an eye on this page: http://downloads.yahoo.com/firefox/

Since, Firefox releases are quicker now - alot of things will lag behind now.

Opps, seem like yahoo identifies FF9 as FF8 still, but here is a better link:

http://downloads.yahoo.com/firefox/sysreq

Wow Yahoo finally updated their System Requirements page for Firefox as they used to have the Firefox 3.6 and earlier requirements listed under Mac for the Firefox 4.0 and newer versions.