The new beta version features stability improvements
Developers also confirmed that they have implemented CSS3 Flexbox and added lots of stability improvements, which have yet to be detailed.
Firefox beta 5 18.0 is now available as a free download via Google Play Store and it should be fully compatible with all devices running Android 2.0 and later. Check it out here.
Keep in mind that if you want a better browsing experience then you should download the stable version of Firefox.