re :can I correct the double spacing every time I do push Enter?

sounds like you are composing in Paragraph mode which uses double space. 'Body Text' uses single space.

• Menu app icon > Preferences > Composition

Under 'HTML Style' section

• uncheck/do not select the checkbox 'Use Paragraph format instead of Body Text by default'

This will take effect in the next Write window you open, but will not auto change in any previously opened Write window.

re ;When typing an email there doesnt seem to be a right hand margin - text just goes off screen

Check setting:

• Menu app icon > Preferences > General
• Scroll to the bottom and click on 'Config Editor' button
• In top search type: wrap
• look for this line: mail.compose.wrap_to_window_width
• If it says: 'false' then click on toggle icon on the right side to swap it to 'True'.

Followed great instructions but was no line exactly as quoted above. Tried changing a couple of similar options but no joy. Close but not really sorted. Thanks for trying P

The defaults in config editor support wrapping, so that shouldn't normally be a concern. There is a rare possibility that you have mailnews.wraplength set to a minus number; that would cause this. Also, TB defaults to wrapping composed text. My suggestion is to reinstall TB. This was a bug several years ago, but I can't reproduce it here.