This is why you turn off these additional features (tools) and keep it simple as an antivirus program only. It all is the best free service available!
The virusscanner of the Czech antivirus company Avast has accidentally been removing Firefox add-ons the last couple of days, even if they aren’t malicious. The culprit is Browser Cleanup, a feature of the virusscanner meant to remove unwanted toolbars and add-ons. In this case the antivirus software also removed add-ons which it shouldn’t have. Users are complaining on the Avast Forums how the software removed some of their add-ons that are harmless.
Avast recommends users to not allow Browser Cleanup to start automatically. The issues are likely caused by the latest Firefox release that also caused problems with several add-ons. However it can’t be ruled out the issue is caused by Avast, earlier this year users also complained about the Browser Cleanup feature removing Firefox add-ons.