Try

Tools|Options|Composition|General

Untick "Use Paragraph format instead of Body Text by default".

Let us know if this helps. Or if it doesn't.

Have tried the above solution to single spacing emails, but every time I reload Thunderbird it goes back to double spacing????

Thunderbird opens just like you closed it unless you have a cleaner software like CCleaner that likes to erase your prefs files.

Well, mine doesn't and I haven't had CCleaner in my pc for a long time.

Is there still no solution to this problem of Thunderbird reverting to double spacing in outgoing emails?? It is very annoying to have to re-set the spacing every day when I open Thunderbird. David

Start Thunderbird in Safe mode by holding the shift key when launching it. Work any better.

Went into Safe Mode, unclicked the relevant box........exited Thunderbird, restarted it normally.....box was checked again.........:-( david

The real test would be uncheck the box and restart in safe mode.

I did that.......in safe mode the box was checked. I unchecked it and restarted in normal mode........box was still checked........:-( dave

Not even close to what I said. Good luck.

Can you explain it for a novice??

None of these solutions work for me, but I do have a work-around. Open your word processor (I use Word) and compose your reply using all of the word processor's editing features. Then, select-all (Control +a) and copy (control + c).

Next, set up the Write header and place the cursor in the body. Right click and select "Paste without formatting". Then you can add whatever formatting you want.

EdS

None of the above worked for me either.....Thanks for the suggestion, but toooo complicated for just a plain email.

Hi dav936, Zenos's post was marked as the solution by the person who asked the original question (tlc5408). Your issue may have similar symptoms, but it is likely a different cause/solution. You should use https://support.mozilla.org/en-US/questions/new where volunteers can get more details about your setup.

Because this thread is solved and more people might start using it for their own questions, I'm going to lock it.