From MozillaZine Knowledge Base

Even though this problem was supposedly fixed in Firerfox 2.0.0.4, it is still being reported, possibly because of a problem with a mismatched user agent. See bug 386518 and bug 386085 - both show rv:1.8.1.3 Gecko/20070309 Firefox/2.0.0.4. See this forum post for a report of a repeated Software Update "loop" in Firefox 2.0.0.5. Alice 01:41, 27 July 2007 (UTC)

Just wanted to add that The mismatched UA issue was also brought up earlier in Bug 374140 – Software Update (Firefox 2001 -> 2002) Problem - wrong User Agents which is still an open bug. Alice 14:28, 27 July 2007 (UTC)