Firefox 19 is released today and will add an integrated PDF reader. Since Mozilla moved to faster iterations of Firefox, like Chrome, we rarely see big changes in updates, so a PDF reader is big news. You can already download it here for Windows and Mac.
From the Firefox beta notes, we can see the biggest changes are the new built-in PDF viewer and some startup performance bugs. First impressions are that startup is faster: Firefox now seems to open almost as quickly as Chrome, which is a great improvement.