Steps to reproduce:
1. Open the attached project (New Unity Project.zip)
2. Go to Build Settings, switch platform to WebGL
3. Build & Run the project for WebGL
4. Open the project in Firefox and verify WebGL2 is used
5. Observe the scene completely black

Note:
This bug only appears in Firefox (v. 50.1.0, WebGL2 enabled)

Expected result: Directional light should work when shadows are enabled
Actual result: Directional light not working when shadows are enabled

Reproduced with: 5.5.0a1, 5.5.0f3, 5.5.0p3, 5.6.0b2
Not reproduced with: 5.3.7p3, 5.4.3p4
Regression introduced in 5.5.0a1