How to avoid reformatting of text when copying to the clipboard?

Whenever I copy text from a email that I am writing in Thunderbird and I paste it into an application (including Thunderbird itself), the text that I get from the pasting is not the text that I see into Thunderbird, because some (bad) reformatting has occurred: quoted passages -- with a "> " prefix for each line -- have been joined into a single line (including the "> " prefix), and double spaces at the end of sentences have been replaced by single spaces (this only in the text that I had written, not in the quoted text). This happens with both HTML and plain-text mails. It does not happen if I copy text from an email that I am just reading (not writing). At first, I thought that the "format=flowed" in the "Content-Type" of the original email made some difference, but this is not the case. I am using Thunderbird 38.0.1 on GNU/Linux with GNOME. Any help? Thank you.