If you use Firefox on your Mac, and don’t have Adobe Reader installed (which is probably most of you, as the Preview app that comes with your Mac works well as a PDF reader), then you probably won’t have any way of viewing PDF files within the Firefox window itself. If you come across a PDF file, you’ll have to download it and then open it in Preview.
In the past, there was an extension called firefox-mac-pdf which solved this problem, but it only worked with Firefox 3 and not subsequent versions and has since been abandoned. So for those of us on Firefox 10, thankfully there’s finally a solution in the form of pdf.js.
I haven’t tested it much, but as you can see from the screenshot it does work, and surprisingly well, although there are some reports of minor bugs. It’s being developed by Mozilla Labs, and the source code is on github where development seems quite active. You can also use the source code to make an extension for Chrome.