webaudio not working on FF

It seems like recent AudioContext support from Firefox broke dancer.js behavior, making `Dancer._getAdapter` returns the webkit adapter on FF instead of the moz adapter. The problem is caused from an assumption in `Dancer.isSupported`, where the existence of `window.AudioContext` is considered as a non-moz browser.

I wraped up this issue by checking first for moz browsers instead of webkit, this way everything went back to normal.