Avast detects URL Blacklist malware when I start Firefox

Avast Online Security (browser extension)
1

Avast Free Antivirus has started to give warnings as soon as I open Firefox. Started about a week ago.

Both Firefox and Avast are regularly and automatically updated. I still use Win7.
What should I do?

Here is a screenshot:

2

It may be advisable to also clear your browser cache and cookies, including 3rd party cookies and restart your browser.

3

Try disabling Firefox add-on’s/extensions and see if that helps.
If it does, you can check add-on’s/extensions are those you expect to see. Then enable one-by-one and test by process of elimination.