The ideal answer is, don't try to paste from Word to Thunderbird. Here's why - Word is writing a word processing document with a lot of hidden formatting, Thunderbird writes in HTML and none of that formatting will work - at all. It just does not translate into HTML format, makes a dreadful mess of what you send, and each recipient may see it differently than you intended. All the embedded coding from Word also clogs up your email silently and invisibly. Yuk! If you are interested, here are two experts providing more information about that https://support.mozilla.org/en-US/questions/1105408 So what to do instead? I suggest that if you really like to compose messages in a different program, use Notepad, and copy as plain text with no formatting into Thunderbird, then add formatting if needed using the Thunderbird compose editor. Or, if you are sending a document where the formatting matters, save it and add it as Word attachment to the email you send, or better still as a pdf, in case the recipient can't open Word documents. I hope these suggestions are of help.

Thanks very much for your reply Agnes I'm using Windows 10 and have a another silly question for you I'm afraid, is Notepad a program I will need to download ? can you tell me where to find it ?

Also another similar question ... when I compose emails in Thunderbird I often find that whenever I hit the enter key I get a blank line and I'm about to type a new paragraph . So in effect it returns 2 line spaces. Can you tell me how to just return to the next line when I hit the enter key, it's just the way I tend to type things often in my business as a live musician. Thanks very much for your assistance again

Notepad has been part of Windows for a very long time... Did you try typing notepad into the search in Win10 bottom toolbar? or looking for it in All Programs - Windows Accessories? or ask Cortana? Your choice. 'Enter' and paragraph spacing - if you copied and pasted from Word, some formatting applying double space might come with it. A trick to use is to 'paste without formatting' or use [ctrl][shift]V to paste, whenever you paste text into Thunderbird. Also, in Thunderbird itself there are formatting choices. Check in Tools - Options - Composition - General and uncheck 'use paragraph format...' A useful trick to try in any program that makes double spaced paragraphs is [shift][enter] - see if that gives a single new line.