Mozilla has released Firefox Aurora on Google Play. In case you aren’t aware, the Aurora builds are nightly versions, which is basically the developer channel.
Usually, new features in the browser are tested in Aurora, before they are released for the the beta channel. It’s sort of like Google Chrome Canary builds.
And as such, Firefox Aurora is not stable enough for daily use, for the majority of users, which is why Mozilla has marked it as “unreleased” on the Google Play Store. But you can always install it alongside Firefox and Firefox for Android Beta.
So give it a shot, download Firefox Aurora from Google Play
[appbox googleplay org.mozilla.fennec_aurora]
Note: Apps and games marked “unreleased” will not let you post reviews on the Play Store. But you can still send feedback to the developer using the review system.