I cannot reproduce it with "Mozilla/5.0 (Windows; U; Windows NT 5.1; bg; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4". Are you sure you tested it on a clean install? Because I observed the same behaviour several weeks ago and I found it vanished after deactivating the "LiveHTTPHeaders" extension.
Mail converted by mhonarc 2.6.15
This archive provided courtesy of JSW4.NET, Internet Hosting Services for Small Business.