Desktop Sungeon Doesn't work within Waterfox 10.0 Browser, A Firefox 10 (64 Bit ) Variant. I believe that this is because the unity plugin is currently only 32 bit. This may cause problems in the future as more people migrate to a pure 64 bit Browser application.
I've switched back to Firefox 10 to play DD for now.
It appears Unity 3.5 will have 64 bit support but not confirmed if that also includes the Browser plug in.
Since the Non Beta will be an application and not require a Browser to run, this may be a moot point in a few months.