The Firefox 18 beta is available through Mozilla's website for desktops, and through Google Play for Android. Keep in mind that the desktop beta replaces any stable release you've installed, while the Android beta runs in tandem.
Desktop version's minor improvements
For desktops, the new Firefox beta has some other minor improvements, such as a way to disable insecure content on HTTPS websites, improved responsiveness for proxy users, performance improvements when switching tabs, and preliminary support for WebRTC, an open framework for real-time communications such as voice and video chat.
The desktop beta also includes support for W3C touch events--essentially, Web standards intended to allow the same touch capabilities as native apps--in place of Mozilla's own MozTouch events.
Windows 8 Modern style version MIA
However, the Modern-style version of Firefox for Windows 8 that appeared in a nightly build last month is not part of the new beta release. The Modern-style app was an offshoot of Mozilla's standard nightly builds, so it was never a given that the next beta would include it. I've contacted Mozilla to see when the app will be ready for beta, but haven't heard back yet.
You can still try the Modern-style Firefox app by downloading a newer version of the nightly build (firefox-20.0a1.en-US.win32.installer.exe) but, like its predecessors, it's still buggy. Mouse scrolling works now, but it was unbearably sluggish when I tried it on a Lenovo Yoga. As I wrote in my preview of the app last month, it does have a lot of potential, and users may appreciate its ability to keep open tabs visible at all times.
For Android users, the new Firefox beta includes search suggestions, but only to users who opt-in when starting the browser for the first time. The beta also now warns users when they visit websites suspected of malware of phishing attacks, and provides an option to exit before loading the site.