It is not a good idea to copy paste anything that has been created in a Word Processing program because it will include a massive amount of unnecessary coding that bloats the email. It can also influence the css html when someone wants to reply to the email. You also need to use fonts etc that is most likely to be available on other computers and OS.

You could try using 'Edit' > 'Paste without formatting' to insert the text, but surely it is just as easy to type the small amount of info that would be in a signature.

Suggestion - create a proper signature html file using Thunderbird. Open the .docx document and save the image as a separate jpeg file on your computer. I created a folder called 'TB sigs' on my desktop and it holds all images I use in signatures and all the different html signature files.

In Thunderbird click on 'Write' Using the Formatting Bar, choose all required formatting no matter what the default says, you need to select all the formatting. If necessary, you can use 'Tables' to help organise separate text sections in the various cells and rows. Create the signature, typing whatever text is required. Inserting the image in the appropriate position.

When finished, save the file as a HTML file in the same folder where you saved the image.

Then tell thunderbird to use that particular HTML file for the signature. See image below as guide. Right click on mail account in Folder pane and select 'Settings'. select: 'attach the signature from a file instead' click on 'choose' locate the html signature file you just saved on your computer. click on 'Open' click on 'OK'

updated to include additional image as aid.

Thanks very much, Toad-Hall. It took me a while to figure out how to follow your directions, but that was me, not you. New .sig is in place.