Bug in Thunderbird Newsreader when composing replies
There is a bug in the Thunderbird Newsreader which affects composing replies to messages.
After clicking the Followup button and attempting to reply to a newsgroups message in the middle of quoted text if you click on enter to create a new line in the middle of the quoted text, type the first words of the reply and then delete a miss-typed character Thunderbird will add a new line to the bottom of the quoted text above your signature. If you delete more characters it will add more new lines to the bottom and then finally if you delete the new line you entered it will move the typing prompt to the bottom of the quoted text after the extra new lines it previously added.
This bug is present in 138.0 (64-bit) and in the previous version and the version before that and probably earlier versions too. The big did not exist in version 115.8.1.