To reproduce:
1) Create new project
Optional: Create a moving cube to better see when it freezes
2) Change platform to WebGL
3) Enable 'Development build' and 'Autoconnect to Profiler' options in Build settings
4) Build and run
5) After 15-60 seconds the browser becomes not responding.

Notes: Sometimes the browser unstucks itself after some time, or if on Firefox, when this prompt appears ( http://i.imgur.com/mNOvz8P.png ), click Continue and it will work again until next freeze up. Trying to debug loads up this - http://i.imgur.com/BRI5CqY.png - but the browser still becomes unresponsive from time to time.

Doesn't happen in Android IL2CPP build

Tested and fails on:
-Windows
--Firefox 45, 43.0.2, 40.0.2, 37.0
--Chrome 48, 50,
--EdgeHTML 13

-OSX
--Chrome 50 (after several minutes)

Works well with latest Safari + FF on OSX.