Very cool idea. I have started messing around with it and noticed one serious problem - it doesn't refresh after hitting "Load URL" if you have saved a new local file over your old one.

I had an error in my page (it was trying to load the wrong filename for the next frame) and I eventually just clipped it to the first frame and attempted everything I could to load it. It kept immediately opening my page in FireFox before displaying anything and in the html FireFox was showing were the links to my next frame. Finally I had to close the widget, open a new one, and re-enter the location of my first frame with file:///somefolder/test/test.html and it worked like a charm.

If you get wonky results, I suggest closing that instance of the widget and open a new one and try again.

Keep up working on this though - definitely a neat idea.