Super dump question about debug in HTML5

Hi Forum,
Sorry this is the first time ever for me trying to do something for HTML5, I have some custom slate widgets working on my game, but sadly my game is crashing thanks to one of those when I launch my game in FireFox x64, all it says I have a index out of bounds, but without any further details, it is super hard to know, since my slates clases are big, so my question is simple, where the hell can I find the log file for HTML5 and see the crash line? or breakpoints or something :stuck_out_tongue:

Thanks guys!