Hi,
Mozilla staff, and contributors like me, do our utmost to ensure the software we make helps our users get the most from the web. As part of this, we rigorously test each version of Firefox internally prior to release. Our goal is to make products that our users love, so nothing beats real-world feedback from the people we care about most.
To accomplish this, we make pre-Release builds of Firefox available so that early adopters can try out new features and report bugs. Users of Firefox Nightly, Beta or Developer edition for desktop are doing just that, and we really appreciate their help. These people are comfortable with software that is still being worked on and may unresolved issues; they are keen to report those issues back so that they can be fixed.
Earlier this year, we launched Firefox Preview and the feedback we’ve received from our users has been incredibly helpful - we really appreciate it. Direct feedback and bug reports have helped Firefox Preview improve, but with more work to do, we are making available a separate Nightly version in the Google Play Store.
If you’ve used a Nightly version of Firefox in the past, you should have an idea of what to expect. This version of Firefox Preview updates every day (so be careful to make sure it does not impact any data charges!), and may be unstable. You get the chance to see what is coming before anyone else, we just ask that if you can please report any bugs you find to GitHub, or send an email to firefox-preview-feedback@mozilla.com.
Thank you for helping us to make Firefox Preview even more awesome!