select the table Format > Table > Delete > Table

Or Highlight the entire table and press delete. But as additional formatting was included you may need to highlight from end of good text above the table to beyond the table in order to fully delete.

Note: If you copy text from website or Word etc and want to insert in Write email message but do not want all the formatting etc..just the text. In Write window: 'Edit' > 'Paste without formatting'

General info on creating etc tables in Thunderbird - should you ever need it. https://support.mozilla.org/en-US/kb/tables-format-messages-rows-and-columns

Thank you. I couldn't highlight the table and you have explained why. 'Paste without formatting' put all the text into the top left cell!

re : 'Paste without formatting' put all the text into the top left cell!

You needed to have removed the table first. It would seem you still have a table and you are in one cell and then you pasted with formatting into that cell.

Have you considered closing the 'Write window and not saving it if it is totally wrong. Then you can click on Write and start afresh.

But this time when pasting use the Edit > Paste with formatting option and you should not get any tables as they are part of the formatting.

Thank you. Problem solved.