I got this on my XP machine last night with the latest Microsoft Update. Was able to disable it through add-on's however as reported couldn't get it uninstalled. Followed the below reg hack found through Google and was able to remove it.
1. Regedit.
2. Goto “HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Firefox\extensions”
(or “HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mozilla\Firefox\extensions” for 64-bit versions of Windows
3. You’ll see “{20a82645-c095-46ed-80e3-08825760534b}”. Right click it and click Delete.
4. Restart Firefox.