Stuck key? Does it happen if you delete using mouse or menus instead of keyboard delete key?

Does it happen if you start Thunderbird in safe mode?

 https://support.mozilla.org/en-US/kb/safe-mode

Usually a bad mouse. Do you have a spare to try?

I'm using a mouse to delete. Haven't tried safemode. I can't say that it happens often or regular enough to reliably mess up. When it happens it kind of sneaks up and it's a while before missing emails that need to be followed up on are noticed. It cleared up last time when I turned Mozilla off and restarted it. Could it be corrupt code? I redownloaded Mozilla but the problem creeps back.

It's a pretty old mouse. I'll get a new one.