When you first use NetLibrary on a Macintosh with Firefox, it may not be configured properly to handle the content. There is a third party plug-in that you can install that supports in-browser viewing of PDF files from NetLibrary.
* Download at http://www.schubert-it.com/pluginpdf/
* Open the .dmg file
* Copy PDF Browser Plugin.plugin to the Library/Internet Plug-Ins folder of your hard drive. Once installed, the plug-in should work with Mozilla, Firefox, Camino, Opera, and Netscape without additional configuration.