I suspect that you might not have tried all the suggestions, because few others have your problem. It is more usually encountered with incoming messages that were encoded in some idiosyncratic way by another email client.

Go to your Config Editor, type in wrap to the search filter and note what settings it shows. Repeat using flow, and report back with your findings. We may be able to spot one that is missing or inappropriately set.

I tried changing changing the boolean arguments as per recommendation, restarted thunderbird but it made no difference. I have since set everything back to default and have not been using it until I find a solution.

I just want the text to wrap to window width when composing an email!

Wrap: mail.compose.wrap_to_window_width default boolean false mail.wrap_long_lines default boolean true mailnews.wraplength default integer 72 plain_text.wrap_long_lines default boolean false view_source.wrap_long_lines default boolean false

Flow: layout.css.overflow-clip-box.enabled default boolean false layout.interruptible-reflow.enabled default boolean true mailnews.display.disable_format_flowed_support default boolean false mailnews.send_plaintext_flowed default boolean true

Surely

mail.compose.wrap_to_window_width default boolean false plain_text.wrap_long_lines default boolean false

should logically be set to "true"?

Mine are.