Crucial addon Rated 5 out of 5 stars

I use Mac OS X 10.5.8 my FF 8.0.1 is 32 bit mode .
The same extension discussed below (Nov 1). i just modified it to work with FF 4 till 10 (in 32 bit mode)
Hassle free - Direct download of this extension :

http://www.megafileupload.com/en/file/333667/FirefoxPDFPluginforMacOSX-1-2-0modified--xpi-zip.html

Just open the Zip file and install the Extension
("install add-on from file" in options of add-ons window in FF)