Threat Secured warning will not go away

I accessed a website and the threat warning appeared as a “threat secured”. I closed the message and closed the website.
10 seconds later the message reappeared, and kept coming back even when I closed my (Firefox) browser.
This has happened several times before, since the last or previous AVAST update.

The only way to turn off the message is to REBOOT!!!

Screengrab of message

The only way to turn off the message is to REBOOT!!!!!
Tried to clear your browser search history log ?

Thanks, but not really viable.
I need most of my history.
I did delete recent history, rebooted and will see if that helps.

Clearing history did not help. Only a reboot makes it go away.
Will a re-install of AVAST help? If so, how do I re-install AVAST?

I just reinstalled AVAST and scanned the system; clean report.
Problem still there.