Open at least one tab, than go back to Firefox Preview homescreen and close the app by pressing device back button.
Firefox Preview is closing immediately
Firefox will be closed only by more then two clicks on back button
Can reproduce on Pixel 3
Cc @mcarare not sure if this is related to the new home fragment navigation / scrolling but worth a look?
Issue is gone in this build via GP
Nightly 200330 18:00 (Build #20901803)
38.0.20200329190103, 1341d92fa
GV: 76.0a1-20200327094805
AS: 0.55.0
Verified as fixed on Nightly 4/1 with Google Pixel 4 XL (10), Samsung Galaxy S7 (7).
Note that Fenix is closed immediately after tapping on the back button.