Avast is generally a well-rated and well-regarded antivirus software. Its Browser Cleanup Tool, however, has a bad history of annoying users. Recently, a bug in the tool was causing it to auto delete all Firefox add-ons.
As Techdows reports, that bug has been patched with the latest virus definition (look for number 14118-0 in the Settings > About Avast menu). However, there have also been previous issues with the Browser Cleanup Tool, such as asking to switch your default search provider and homepage to Bing or Yahoo if you run the tool with Internet Explorer, and in my experience, the tool would warn about toolbars and add-ons that aren't actually threats.
Techdows offers the instructions to uninstall the Browser Cleanup tool: Go to Control Panel > Programs and Features > Avast Free Antivirus and click Change. In the setup dialog, remove the "Browser Cleanup" option under "Tools."
Avast has been annoying me recently in other areas, though, such as the constant voice alerts (interruptions) that the virus database has been updated. I'm looking into Avira now, since it also ranks well in antivirus tests, but if you have any other favorites, feel free to share them.