This typically happens when people let CCleaner mess with the Thunderbird profile. If you do that, stop it.

I noted your response about CCleaner so I unticked all the options under Applications/Thunderbird. However the Message Pane continues to appear randomly. What next, please?

haroldf,

you posted to a 'solved' topic. Wrt your problem, please stick to the topic you started at https://support.mozilla.org/en-US/questions/1159617

Solution: I noted when I clicked View>Layout>Message Pane, and it didn't respond to unticking the thing so the Message Pane would stop displaying, it *does* have F8 beside it. Hitting the F8 key turned off the Message Pane. I don't know why I hit F8 while I was in Thunderbird, but either I did or the program just assumed I'd like to preview a message (I never do want to do that).

Hope this helps.