There's a very nice Services add-in called Print Selection from Robert Chin. Unfortunately, its menu item (from the Services submenu) isn't available from within FireFox (or most other Carbon applications). And there doesn't seem to be a 'Print Selection' extension yet for Firefox, either.
The solution is quite simple. Open the top-level /Library/Services menu, and drag the PrintSelection.services application to your Dock. Now simply select some text in Firefox (or any other non-services-aware application), and drag it to the icon in the Dock. Works like a charm.
[robg adds: I tested this and it worked as described.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051201121241531