I can imagine this happens when replying to a message but hardly when starting a new message. Can you confirm this?

Christi, it's true. Starting a new e-mail, this is what comes up on the letter. It goes on and on and on! I know it sounds crazy and I've been trying to find out how to get rid of it ever since I started using Thunderbird about a week ago.

Any help you can give - even just suggestions - would be greatly appreciated!

There are several ways in which pre-defined content can be inserted into a message. Typically, you'd use a template, a draft, a Stationery template or a signature.

It seems unlikely that a draft or template would automatically do this; you generally have to explicitly select the initial document. The Stationery add-on will automatically use the last Stationery template you used, but unless you have installed Stationery and specifically created a suitable template then this seems unlikely too. So, have you perchance pasted any text into the signature block under Tools|Account Settings|<select account>|Account Settings?

BTW, and in the meantime, ctrl+a in the new message will select all, making it easy to delete this prefilled stuff.

Do you use any other add-ons?

I have absolutely no idea what I did, but everything's gone except for the -- (two dashes) which I can definitely live with.

Thank you so very much for taking the time to help me. I do appreciate it very much!