That's really strange behavior. It would be very helpful if you provide us more info so we could analyze the problem.
(1) Run avast!.app and go to the WebRep page.
(2) It should say you that it is installed in your case for the particular browser. Is it true, or dies it misdetect it?
(3) Click the button "Uninstall" if it is there (if not try install and uninstall).
(4) Either installing or uninstalling the browser plug-in generates a log /tmp/avastinstusrscript-XXX.log,
where XXX is some number. Please provide us the two (!) latest log files. This should provide is enouth info
to see what;s wrong in your case and provide you some advice and fix it in next versions.
Thanks for the report.