Huh, practicing should definitely work also on Firefox. Could you give some more details. Does it work for you with Chrome? Does it work when you try the practice demo on the home page directly at keycombiner.com?
The only limitation with Firefox is that it does not support the navigator.keyboard API. So, the visual keyboard and some other advanced KeyCombiner features cannot show your system's keyboard layout but rather have to default to the US layout.
I deployed two minor (unrelated) bug fixes shortly after posting on HN. I thought this works without downtime etc., but maybe you somehow downloaded an incomplete version of the JS code. Not sure how that could happen.
The only limitation with Firefox is that it does not support the navigator.keyboard API. So, the visual keyboard and some other advanced KeyCombiner features cannot show your system's keyboard layout but rather have to default to the US layout.