Firefox has been fast and furiously releasing new versions, foregoing the standard dot releases and going with a "rapid release" ever since 2011. Over the past two years we've gone from version 4 all the way to version 18. The latest version, 19, is expected to be officially released tomorrow and will include the ability to view PDF files natively within the browser.
If you're impatient, you can actually download the latest release right now. As TNW reports, the binaries have already been placed on Mozilla's FTP servers and can be installed on Windows, OSX, and Linux. Although Mozilla announced back in December that they will work on a Windows 64-bit version of the browser, we will apparently have to wait until at least version 20 before that will be available.
There is no changelog or release notes yet, but there is a list of what changes were made in the beta version of the browser, and that should be a good indication of what will make it into the latest formal release.
Let us know your thoughts on version 19 if you decide to upgrade before the official launch.