After the 188.8.131.52 -> 184.108.40.206 major update, if you go to Options(Preferences)/Advanced/Updates "Show History Status" and check the update history, it shows, "No updates installed yet." The update should be shown. This was found on a Windows XP machine. Clean profile with a clean install of 220.127.116.11 before updating to 18.104.22.168.
rhelmer points out in bug #368082: "Hmm.. might this have to do with the directory that updates are stored in changed during the 2.x release line (for better Vista support)?" rob helmer is right on the money. I think this is really a dup of bug #372981
confirmed dupe this is (btw) the same that happen also on the Thunderbird Major Update (Bug 391792)