Because of how Discourse dynamically generates pages, using the Print function in your browser will only ever capture a section of the topic.
Fortunately, Discourse has anticipated this: If you hit CTRL-P (instead of using menu navigation to print), the page will capture that input and create a pop-up window that contains the full text of the thread. (If you have pop-ups turned off, you'll need to manually open this window.)
If the CTRL-P command isn't working for you, you can achieve the same effect by adding /print to the end of the URL.
And you'll get the print-friendly version of the page (and probably a print dialogue).
This works in both Firefox and Chrome.
-- Justin Alexander