Firefox 18 has just been released, but Mozilla are working fast to publish their first Beta release for the upcoming Firefox 19. One of the key feature of Firefox 19 that has been expected by many people (including me) is the native PDF reader without any need to install PDF plugins by Adobe Reader.
This feature has been included, but disabled by default by the developer since it's still considered unstable (but can be enabled by modifying the about:config window). This time, the developer thinks it's ready for public use.
Another interesting features are the new experimental Remote Web Console which can connect to Firefox for Android
and Firefox OS web applications and interact with them from within the
desktop installation of Firefox and Browser Debugger for add-on and browser developers. They are still disabled by default for now and probably be enabled by Firefox 20 or even 21.