Email generally doesn't have settings for margins, so you will on a somewhat futile error looking for any settings in Thunderbird to control them. The only settings I'd expect to find in an unmodified version of Thunderbird would be related to printing on paper.

However, document processing software does know about margins. In your case, I'd be looking at the source of the incoming message to see what formatting has been applied. I have seen some bizarre markup, where an email message has been formatted as if it were to be printed by a word processor, with margins, indents, right-justification, the works. All this is a poor fit to the intended use of email.

There are add-ons like Stationery which allow you to define some formatting, and this can indeed apply margins, justification and so on. But right now, i think you need to remove the exsiting formatting, not add even more into the mix.

Given what you're describing, I'd try working around it by copying the text of the original message and then use "Paste without formatting" as a way of stripping out the formatting that came with it. It might then behave more sensibly.

Alternatively, copy that text, paste it into a plain text environment such as Notepad (doing this will again remove all the hidden formatting), then copy and paste it back into your email message, replacing the original text.

Many thanks!