Heard that addon's name come up a couple of times as malware.

CHECK WITH ALL PROGRAMS BELOW

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

In the Windows 7 Control Panel, Uninstall a Program, you can click the Install Date column to sort by date and look over your recent installs carefully. Often you will find a number of unexpected programs with bland names designed to disguise what they do.

Based on past threads about Websteroids / ScorpionSaver / Tube Dimmer, you might find the malware that does the reinstallation in this hidden folder:

C:\ProgramData\Updater

If Windows won't let you delete an .exe file in that folder, you may need to kill the corresponding process in the Windows Task Manager. In some cases, it's easier to restart Windows in Windows Safe Mode and then run the cleanup programs listed in the first reply.