Gets stuck for me too on 68.0.2 (64-bit/Linux). Works in Chrome. Looking at the console I get:

<SetDimensions>: Failed to create WebGL context: WebGL creation failed: * Refused to create native OpenGL context because of blacklist entry: FEATURE_FAILURE_GLXTEST_FAILED * Exhausted GL driver options.

This is my work laptop so I never bothered to setup Mesa/GL correctly .. probably the issue. The app should still do a check and report that WebGL isn't enable to the user.