I only noticed it in FF15, but it may have been there for longer. Change pdfjs.disabled to false in about:config (followed by a browser restart) if you want to turn it on. Here's a test file[0] should you wish to test it out.

I've been using pdf.js for a while on my Windows box (where it is enabled by default on the FF beta channel) and it has performed remarkably well. Sometimes it's a bit slower than Adobe Reader or messes up colour or formatting, but on the whole I've quite enjoyed using it.

[0]: https://svn.torproject.org/svn/projects/design-paper/tor-des...

Note that the text in [0] doesn't scale properly, and the page previews in the navigation pane just show garbled text. Usable, but only just.