Found it be caused by a McAfee file, and update one I think? See https://community.mcafee.com/thread/74287, been around for awhile. Not everyone gets it either.... :(

This was an upgrade over W8.1.

Only basically happens once, shortly after booting. Only time Thunderbird came to the front was when I was doing a reply. Otherwise I can't tell where the focus...

I'm also seeing something stealing focus, but only ONCE after booting.

I too could be browsing something and all of a sudden the mouse wheel no long will scroll the page? Clicking on Firefox will...