Starting from Firefox 19 (i think) the built in PDF viewer will be enabled for everyone. What annoys me about this is the stupid yellow notification bar that says "This PDF file may not be displayed correctly"
Even examined it a little bit with DOM Reader but no luck getting rid of the bar. Can anyone figure out the code?
If youre using current stable 18.0.1 then you can enable the viewer from about:config and setting "pdfjs.disabled" to false.