Do you only have a problem in the 'Write' message compose window ? OR do you have same issues in other areas such as Message List or Folder Pane or when reading emails in Message Pane ?

If this is only when you compose emails, is the just the font size is too small OR is the font size perfectly ok but there is no spacing between the lines of text?

Post an image showing the problem.

Does this occur when using 'Reply' or just when using a new Write message or both?

Please confirm you have this setting before doing anything else.

• Settings > Composition
• Under 'HTML Style'
• Size : select 'Medium'

Post the following information: In Thunderbird

• Help > Troubleshooting Information
• Scroll down to 'Important Modified Preferences'
• Look for lines that start with 'font'
• Copy all those lines and paste them into this forum question.

I found the trick, thought I'd share it for posterity: do all the customary advice, get it how you want it otherwise, but to control Line Spacing upon hitting CR/LF (enter), to avoid the giant double spaced lines of text, check to ensure you have the composer set to use 'Body Text'. It's right there at the top of the composition's window, on the left. For unknown reasons it's now set to 'Paragraph' which gives the double-line spacing that drives people nuts! I'd guess, if I had to, that the 'Paragraph' (double line spacing) setting is an artifact of choosing to compose in HTML (it might have been set that way on installation or a choice made in setup....)

So that's the trick: compose with 'Body Text' selected. You can select text that's double spaced by highlighting it and changing that setting to whatever you like, experiment, go hog-wild! Other font and composition choices apply the same way including font size, color, etc. There's a row of buttons to play with, enjoy.