Firefox 3 browser does not allow to open .pdf documents directly, not at least for Mac users(this is what i see in my macbook). Here’s when you need some tool that supports .pdf in firefox browser and a google code has seemingly done the job for everyone.

Its firefox add-on named firefox-mac-pdf which when installed, helps firefox users open and read .pdf documents directly in the browser, rather than downloading it and viewing in adobe reader, which is double pain for everyone. So here is what you need to view pdf files in firefox -

Download firefox mac pdf add-on

After you download and install the add-on, just restart the browser and try opening any online pdf document. You won’t see any save file box out there, but you would see the document open directly. The add-on uses PDFkit code to display the pdf files in the browser.

Not sure whether it is the same for windows or not, so will update if its the same or not.