hello, short answer is: you cannot in the pdf viewer that's built into firefox - please refer to bug #792816. in general you shouldn't rely on the print restriction in adobe as a "security feature", since in essence all it does is add a flag to a file, that says "please don't print me" to an application - this is very easy to circumvent and many third-party pdf applications won't respect that flag.

Thanks for the info. Is there any other way to prevent PDF files from being printed?

There are always ways to download and save the PDF file and open the file in a PDF Viewer that ignore the print protection. This feature is probably only supported by the Adobe Reader program and won't work as well on other platforms that have other PDF viewers like Evince on Linux and Preview on Mac.