Copy of the message is here:
(The link modified - doesn't go anywhere.)
What I'd be inclined to do is scan for outdated software, in order to check your flash player etc is properly up to date. This can be done from the Avast scan interface.
Update anything indicated.
Then scan for undesirable browser add-ons. Remove anything flagged.
Download adwCleaner from here
https://toolslib.net/downloads/viewd.../1-adwcleaner/ and run it.
Set Firefox to block undesirable sites/processes:
Open "tools" in Firefox. (Should be an icon shaped like a cog.)
In the "Security" tab, put a tick in the first three boxes, namely "warn me when sites try to install add-ons", and both boxes under and including "Block dangerous and deceptive content".
Restart.
Any better? Anything found by any of the scans?