Got caught by the same problem. Latest angularfire2 requires rxjs6 which doesn’t work with ionic 3.
Angularfire2 rc.5 (or rc.6 as far as i remember) was the last one with rxjs5 support (which ionic 3 uses), but pining to an old version doesn’t seem like a good option (no bug fixes).
I tried rxjs-compat but couldn’t get it working either, so I tried to do the ‘jump’ and test ionic 4 + angular 6 (which already supports rxjs6 for angularfire2) but I got stuck again trying to run the simplest starter on a real device.
Let’s see if someone can help here