Please provide more details. For example, if you are pasting HTML or Word documents, they contain their own formatting which can follow to Thunderbird.

I can't reproduce it just now. When I can, how do I send it to you?

You post on this forum. In this topic! It will remain open for 6 months waiting for you.

Fundamentally you have to look at the source of your text before you even think about complaining. Microsoft Word and other application software generally classed as "Word processors" are designed to place marks on paper. They are mostly very good with old fashioned typography. Not so much with modern copy and paste as the capability of the destination is unknown.

A good example of that is when you paste from Microsoft Word. I was playing with the process for another support topic just a couple of days ago. Pasting from word a small sentence "Some silly text" from word to Thunderbird with a hyperlink under silly. In plain text terms 16 characters was placed on the system clipboard by word for me to paste. However Thunderbird if it has a choice will paste formatted HTML text. Sure enough word had placed the information on the clipboard also as HTML. A block of code 39,633 character long was what was offered. Including extensive cascading style sheets that were not even used. So just because there is an issue in Thunderbird the source might not be Thunderbird.

Just to be sort of complete, word also placed rich text formatted text, plain text, an enhanced meta file image of the text and a binary object that could be used for object linking and embedding among others. Pressing copy is an apparently simple function, but what happens under the hood is complex and can involve a lot of moving parts.