You could visit the Avast Forums and see if others have gotten this and if they've determined it's a FP.
No other tabs and Edge the only open program.
It could could be a ploy to get you to upgrade to the paid version. Or as mentioned, a false positive.
Your other option is to d/l Malwarebytes free & do a scan with that & see what it finds. Be sure to unclick the "start free pro trial" box at the end of the installation. It wouldn't hurt to get a "second opinion" so to speak. And having another layer of protection never hurts either
Malwarebytes | Free Anti-Malware & Internet Security Software
I already have Malwarebytes and ran a scan after this occurred without a detection. I've had this come up a couple times now, both times coming out a sleep with nothing but Edge opened to tenforums. I'm not too worried about my system considering it was detected and blocked, was just concerned something iffy was going around these forums.
Well, if you want to check into it further, there is another program you can run that is good at ferreting out adware. It's called AdwCleaner.