TraceyC For Thunderbird, you can make a copy of your profile folder
Thanks, Tracey. Before every update I do a backup of my profile folder, which contains a list of all the packages I've installed. But in this case, I have no idea how far back to go so as not to restore things that caused my problem in the first place. So, before I saw your message, I'd already decided to reinstall the OS on my laptop.
I'm nearly done with that now. I'm just configuring Vivaldi - creating my tab stacks and such - and I've already completely configured Thunderbird. I just need to finish configuring Vivaldi, and I'll be back to where I was before all this happened. I'll be re-creating email filters for a while, but I can live with that.
Thanks again for your advice, which is always good. This time, the situation sent me in a different direction.