Rated 1 out of 5 stars

Try pdf.js extension. It works for 64 bit FF.

search add on on this website or here is a link:

https://addons.mozilla.org/en-US/firefox/addon/pdfjs/

Rated 3 out of 5 stars

This addon is really nice for 32 bit but if you want a pdf viewer for firefox 5 in 64 bit try PDF Browser Plugin. Here is the link:

http://www.macupdate.com/app/mac/9542/pdf-browser-plugin