That's why I always uninstall programs with Revo Uninstaller, to make sure programs are gone without leaving annoying remnants, even so I still sometimes find them here and there. Maybe you have remnant Avast add-on, in Firefox Tools - Add-ons? Or try to start browser in safe mode with add-ons disabled - it restores browser to default settings, in Firefox go to about:support - Try safe mode (Restart with add-ons disabled), and see if anything changes.