Google introduced a new way for developers to package web applications into Android apps last year, called Trusted Web Activities. The web apps run inside a modified Chrome Custom Tab, which meant that if your default browser wasn't Chrome (or based on Chrome), they would default to running in Google's browser. Firefox for Android has now added support for TWAs, so if Firefox is your default browser, the apps will run inside a Firefox container instead of Chrome.
The functionality has been live in the Firefox Nightly channel for a little while now, and should arrive in the slightly-more-stable Beta channel soon. If you have Firefox Nightly set as your default browser, and you install an app from the Play Store that is packaged as a Trusted Web Activity (like Google Maps Go or PhotoStack), they should open in Firefox.
PhotoStack TWA running in Firefox
For the moment, Firefox doesn't hide the bottom address bar like Chrome does, but that's a bug that should be fixed soon. Hopefully, Mozilla will smooth out the rough edges over the coming weeks and months.
- Tiger Oakes (Twitter)