Step 1 – Open your Thunderbird eMail application. Mine is on the Task Bar at the bottom but yours might be on your desktop. Just look for the Thunderbird icon.
Step 2 – Head to the top and select “Tools”
Step 3 – After the drop-down menu appears go to the bottom and choose “Options”
Step 4 – Select the “Composition” option at the top
Step 5 – In here you will notice a check in the box next to, “When using paragraph format, the enter key creates a new paragraph”. Uncheck that box.
Step 6 – Last thing to do is hit “OK”
Now you will be able to hit the Enter Key and just be moved down one line, not two. This is the way I fixed my issue, so if you know another way, please share in the comments down below.