If you want to open PDF files inside Firefox you need to install mozplugger.mozplugger allows you to seamlessly integrate external applications to view files downloaded from the web that Mozilla can not normally handle. The application is embedded within a Mozilla window as to act like and feel like a true plugin.
This allows you to view PDFs, Postscript files, animations and movies, amongst other file types all from within Mozilla (with supporting applications).
Install mozplugger in ubuntu
sudo apt-get install mozplugger
That's it from now on you should be able to view pdf,postscript files
Guys this tip was submitted one of our blog reader and the original source from here