Thanks for trying out GWD. I liked your animation, let me try to answer some of your questions.
First, can you give me some more information about what you mean by blank keyframes? The way that you authored your animation is how I would do it.
For looping the animation you need to set the loop count on each layer. You can do this by clicking on the loop icon which is next to the lock layer icon and layer visibility icon on each layer. This will bring up a UI for changing the animation loop count.
To preview the animation on Firefox you need to use the "Preview" button in GWD and then select Firefox as the browser. This will add the appropriate browser prefixes so that the animation will work in Firefox. Once you are happy with the preview you can then publish your file. This will create a document for you with the browser prefixed and unprefixed animations.