Reproduction steps:
1. Open the attached project;
2. Open the scene "Demo";
3. Enter and exit Play mode until Unity crashes;

Actual result: Unity crashes in <Firebase.FirebaseApp/FirebaseHandler:Update>.
Expected result: Unity should not crash.

Note: Unity crashes in Update() function which is in FirebaseHandler which is created in runtime. Sometimes Unity does not crash, but Editor freezes.

Reproduced with: 5.5.0b1, 5.5.3p3, 5.6.1f1, 2017.1.0b4

Cannot be tested with 5.4 because of many errors.

Note#2: "Firebase" developer solved the issue. The fix is in the latest version of "Firebase" Unity package: https://firebase.google.com/support/release-notes/unity#3.0.3