I am on Mac. Up to Firefox 18 I used this plug-on: http://code.google.com/p/firefox-mac-pdf/
I had to change though the maximum version number inside the xpi file so that it is still usable, but I have found this plugin way superior to anything else. Since Firefox 18 I cannot use it -- even though I set the max version number above 100 When I try to install my tweaked version, Firefox just reports that it is not compatible without any further information.
Unfortunately, the new built-in viewer is not a good solution for me as it has all kind of font rendering problems, especially when it comes to printing (I am mostly reading and printing math).
Any help would be much appreciated or, sadly, I will need to forget about using Firefox. I wasted quite a few sheets of papers printing gibberish as I keep forgetting that printing just does not work from the browser.