1)Yeah the fullscreen seems to be a work in progress but I can't say. You can always use your browser's zoom to get a bigger window though. This may make the graphics blurry, but if you use Chrome you can go to chrome://flags and enable "GPU Accelerated Canvas 2D" which keeps graphics from blurring, among other things.
2)The game I'm working on uses large images, 4 parallaxing layers, lots of little objects, and a fair amount of particle effects but there's no slowdown yet! I don't think you'll have any problems there.
You may want to find a better browser for gaming; FF isn't very good imo. Maybe Chrome or Opera?