I am guessing you are another of those that think you need to press enter to end a line instead of allowing the document to wrap in the appropriate place depending on the size of the display on which it is being read.

Options/ Preferences > Composition > turn the option for use paragraph style (a line space between paragraphs) to body text (which has none)

Wonderful! That did it. Thanks Matt!

I am guessing you are another of those that think you need to press enter to end a line instead of allowing the document to wrap in the appropriate place depending on the size of the display on which it is being read.

Funny that sentence starts off with condescension, and yet they missed the first three words the OP posted: "On a Mac". The "solution" provided is for the Windows version of Thunderbird...um...

The correct solution for a person on MacOS or Linux is to go to: Edit > Preferences > Composition > "General" Tab, and then uncheck the box that says, "Use paragraph format instead of Body Text by default".

I unchecked the "use Paragraph format..." box however still double spacing. Checked left hand alignment on tool bar and so far it works.

I have tried all of the above suggestions but the double spacing persists. Body Text reverts to Paragraph as soon as a key is hit.

Make the change in options. Close Thunderbird. Restart Thunderbird. Click to write.

Did the change stay set when you closed down. If not you probably have some other software on your computer preventing Thunderbird saving changes.

Try the following to try and narrow down the cause of the problem;

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Thanks, Matt. Changing the paragraph setting in options solved the problem. Windows Live Mail was much simpler!

The problem is when REPLY-ing to a message! (My ver 68.12.0 32-bit). (normal composition with a new email is OK). Double space is the default, and changing it is NOT "sticky"! It reverts to Double-spacing the next time you open Thunderbird! IE; If you un-check the; tools\options\general\ "Use Paragraph format instead of Body Text by default" box, it works correctly (single spaced) for that session only. When you close and open Thunderbird, you have to go in and UN-check the box again. This used to work correctly on prior (several years ago) versions of T-bird. So it was "broken" at some point. Trying to find out when in the release notes is futile. Keith

And no, I have no add-ons. And starting in safe mode I find the box is still checked. Un-checking and re-starting again and it is checked. You have to go in and un-check it EVERY TIME! It is a BUG because that selection is NOT Sticky. Thanks, Keith

This stupid program will NOT single space. Fix the BUG