There is no official changelog available for the moment
Mozilla has just released a new beta version of Firefox for Android devices. This would be the fourth beta since the stable iteration was bumped to 21.0.Unfortunately, developers have not released any changelog for this update, so we must assume it only brings some minor bug fixes that are not worth mentioning.
Keep in mind that this version of Firefox is a work in progress, so users might experience issues, lags or freezes.
Obviously, based on user feedback, Mozilla plans to refine and polish each new version of Firefox and once in a while, it releases another stable version.
Firefox Beta 4 21.0 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 up. Check it out here.