Just noticed that Thunderbird says...
You are using the daily version of Thunderbird, which means you get to experience the absolute newest Thunderbird - fresh out of the build system! But with great power comes great responsibility, we hope you’ll take the time to report bugs and provide feedback on your experience using the daily version and help make the next Thunderbird release as awesome as possible.
And later...The Daily release of Thunderbird can be unstable.
I don't really want great power , especially if it means it could corrupt my local emails, I want stability! I'm not a bleeding edge kind of guy!
So I thought I'd just install the one from their homepage...but that download is the same version[68.4.1]!
Since when is an unstable version...the default download?
My Linux Mint version is 60.9.0, so something older than that and newer than daily would probably be best.
So I googled and found How do I get off Daily for Thunderbird
Well I must be stupid because the answer which seemed to satisfy the questioner was
Open Thunderbird (not daily) On the toolbar > options > Advanced > general and click the check now button.
Umm...what do you mean Open Thunderbird (not daily)? That's the problem, daily is the version that's installed.