See https://support.mozilla.org/en-US/questions/1427439

The referenced discussion doesn't help me. I am an octogenarian tech dinosaur who has used and appreciated Thunderbird for years because a) I was already using Firefox for privacy reasons, b)Thunderbird enabled me to handle web-based e-mail accounts in one place on my desktop, and c) I found it so easy to use. I'm not about to start pretending I'm tech-savvy and poking into the innards of the application. I tried the changes I could find under "View", but they didn't help. I thought that I could revert to a previous version which might not have this problem (I have done this with other apps), and I did find a listing of previous versions, but apparently you have to be tech-savvy to figure out how to install them. I don't know why, if I put the cursor on the line between the message list and the current message itself, it shows the symbol that says you can move the line, but the line doesn't move. I applaud a comment I saw somewhere else on a Thunderbird community page, that new releases shouldn't go out for general use by the non-technical user community until the bugs have been found and worked out (if this were specified as a beta I probably wouldn't have downloaded it). I realize this is a minor problem, but it irritates me again every time I open Thunderbird. I have always felt I should update major applications regularly for security reasons, but after this I'm not updating Thunderbird any more. Additional notes: I finally found the Card View and don't like it at all. I also found that the symbol for moving the line permits you to display more messages in the message list, but not fewer.