"Block pop ups" box keeps mysteriously getting unchecked

The "block pop ups" box for Firefox keeps mysteriously getting unchecked as I surf the web. This is completely bizarre. Why is this happening.

Running XP Home SP3, Avast anti-virus, Ad-Aware, Windows Defender.

Download and run free Malwarebytes, CCleaner and tell us what they find

I uninstalled and reinstalled Firefox. That did the trick.

However now I'm noticing that hyperlinks in emails that I open with Thunderbird don't work. Now I'm reinstalling Thunderbird. Gotta back up my emails with MozBackup first. Strange days indeed...

UPDATE: I uninstalled and reinstalled Thunderbird with my old emails but still the same problem. So next I cleaned the registry and did a "disk cleanup" and one of those two or both fixed it. Yeah!!! Hope this helps someone in the future......