To be perfectly honest, lack of support of Firefox 4.0.x is not a big deal. The differences between Firefox 5
. IMO, Mozilla could've gotten away with versioning Firefox 5 as 4.1 or even 4.5. The only reason they are moving to this rapid release cycle is to catch up with Chrome, IE, and Opera, all of whom are working on versions in the double digits. When Google releases a new version of Chrome, you don't typically see people complaining the old version is unavailable. Now that Firefox is moving to a Chrome-like release schedule, I don't see why they should do things any different. If you do want the old version, do some exploring of Mozilla's FTP site.
UserAgent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0.1) Gecko/20110608 Firefox/4.0.1 SeaMonkey/2.1