While Chrome has had an internal PDF viewer for a couple of years now, Mozilla only just added the feature to Firefox 19. It's a feature that most people will find convenient as is, but if you're looking to take some control over how Firefox handles PDF files on the Web, here's how:
PDF file handling options
The Firefox PDF viewer is enabled by default and will display PDFs within the browser when you click on PDF links. If they don't, make sure you've disabled any PDF plug-ins you may have previously installed. If you prefer to use another PDF viewer, like Adobe Acrobat, you can change the default PDF viewer. Go to Options > Applications, then find the listing for Portable Document Format (PDF). Under the Action column, you'll see that it's set for Preview in Firefox. Click the drop-down list to select an alternate PDF viewer.
You can also choose to save PDF files by default when you click on them, rather than opening them. Or, you can set it to Always ask, so you'll get a prompt every time.
Disable Firefox PDF viewer
If you want to disable the built-in PDF viewer entirely, type, "about:config" in the address bar, then click on the "I'll be careful, I promise!" button. Next, search for pdfjs.disabled, then double-click on the entry, which will turn the boolean value from "false" to "true."