Print selected text in message
From MozillaZine Knowledge Base
Unfortunately, Thunderbird does not normally allow you to print only the text that you have selected in a message. If you try to do so by selecting some text and clicking on the Print button, the option for "Print selection" in the Print dialog is grayed out and cannot be selected; the only choice is to print the whole message or not print it at all.
One workaround is to click "Forward", which will open the message in a new window. If you now select some text and bring up the Print dialog, the "Print selection" option can now be selected, and you can now print the selected text instead of the whole message.