Avast keeps popping up this notification:
I did a full system scan and it didn't find anything. What should I do? Get a different antivirus? When I click on "more details" it takes me to this Avast website.
Evidently that particular url in the "object:" line is passing out malware, no infection on your pc but could be infected if you go to that url (website).
Websites become infected and pass out malware to your pc when you visit them, and most of the time the owner/operator of the website has no clue they have been hacked to serve malware.
If they clean the website of the malware more than likely Avast will stop alerting on that particular URL.
If you do not use waterfox, uninstall it.