While in Add/Remove Programs of the Control Panel, I noticed I had 3 versions of Firefox installed supposedly. Versions 11, 12, and 13.0.1 along with the Mozilla Maintenance Service were listed as installed. We know in the version 12 update a new silent updater was included and I am wondering if the multiple versions and that update are related.
A bug reported five years ago in Firefox 2 points to a similar problem with just a Registry key being left behind. Looking in the same location, nothing was found but keys for Mozilla Firefox 11.0 (x86 en-US) and Mozilla Firefox 12.0 (x86 en-US) under HKLMSOFTWAREWow6432NodeMicrosoftWindowsCurrentVersionUninstall
Other entries for the older versions existed under HKLMSOFTWAREWow6432NodeMozillaMozilla Firefox
The easiest way to deal with the problem was to uninstall Firefox and run the installer on the other versions to clear them from the Add/Remove Programs list. Since I told the uninstaller to not remove my personal settings, I was able to uninstall safely and install Firefox 13.0.1 without losing any preferences or add-ons.
With a new major version of Firefox released every 6 weeks, hopefully the new bug is squashed before the list gets too out of control.