For a couple of years, Firefox Aurora, the developer edition of the popular browser, has only been available directly from Mozilla. As of today, Aurora has landed on the Play Store, in early access (i.e. unreleased) form. This is presumably because it's unstable and not meant to be used by the general public, and only by those interested in what's coming soon or building the web. Regardless, it is always good to see things in development that will later be deployed to the public version of Firefox, which is currently on v49.
I've been trying to find out when Mozilla first released Aurora for Android, but the best I've found is v29.0a2, made available on February 7 2014. Anyway, the version available in the Play Store now is the exact same as that available on Mozilla's Aurora website, v51.0a2, which was released on September 20. v51 doesn't add much, but what it does add is significant: Skia content rendering. Skia is a graphics library that is owned by Google (and so used in Chrome and its variants), but is also open source. From what I can tell, it makes the rendering of certain things faster - if anyone wants to elaborate in the comments, go ahead.
Firefox Aurora is available in the Play Store now, so if you're at all interested in what the Firefox team is working on, go get it. As always, it is still available directly from the Firefox website as well.