If you switched to Firefox from another browser like Safari that supports in-line PDF viewing (that's the ablity to look at a PDF in the browser instead of downloading it), then you might have been disheartened to learn that Firefox does not support this feature natively. However, you will find Firefox-Mac-PDF to be a useful plug-in.
Firefox-Mac-PDF allows for the same in-line PDF viewing that can be found in Safari. This plug-in requires Firefox 3 and Mac OS X 10.4 (Tiger) or higher. You can download the plug-in for free from their Google Code page.
To install the plug-in, just open the Add-ons panel by going to Tools > Add-ons. Once there, drag the downloaded ".xpi" file to the Add-ons. You will be asked if you wish to install the plug-in. Once you restart Firefox you'll be able to view all PDFs right within Firefox.