I've been holding my breath and trying not to put the evil eye on it but, having done absolutely nothing to fix it because I wanted to wait until I got a consensus among you all, it stopped doing it three days ago - shortly after my last post here. SSHHHH!!!
Mozilla still pops up the same message every time I make my first change to Thunderbird after opening the program so I've still got that to put up with but OE has gone back to sleep. Yippeeee!
Thank you all for you efforts. Let's hope that I don't have to raise the subject ever again.....