Recently, Mozilla announced the end of its Test Pilot program, which it has been using as a way to test new features for its products before they make their way to the general public. It looks like the organization has no plans to end public testing on the whole, however, as it today launched a new mobile app called Reference Browser.
Reference Browser is a concoction of elements, such as GeckoView, Glean, and more that are part of a new browser infrastructure, but it's not meant to be a preview of a new product. Mozilla does not intend to ship the Reference Browser as a product later on; instead, the Reference browser combines these components so that they can be tested in conjunction with each other. Some, or all, of these components, may eventually end up on Mozilla's other products, but Reference Browser itself is simply a testing program.
The app is available exclusively for Android, and mobile devices are the focus of the test. If you'd like to be part of the test, you'll need to join this Google group, and then head to the Play Store listing for the Reference browser to download the first Nightly build. Any issues you find can be reported on the GitHub page for the project, but you'll also be sending telemetry data to Mozilla by default. You can turn this off in the settings if you prefer.
For now, the browser may be lacking some features, but you'll already find a lot of the basic functionality. The address bar is at the bottom of the screen for easier access, private browsing is already available, and you can turn on tracking protection for both normal and private browsing tabs. There's also a new implementation for Firefox Accounts, which lets you sync your browsing activity between devices. If you'd like to be a part of Mozilla's browser development, be sure to give it a try.