I've just noticed in FF and Chrome that clicking the "Upload a capx" link in the "Edit Game" section of an uploaded example of mine did not work.
In chrome, there is just a quick loading time, without any pursuit.
In firefox it takes directly to a "Firefox does not know how to execute this URL" page of Gecko itself.
In firefox though, for a second before going to that page, I see a dialog with what I guess is a disclaimer.
Modifying it in firebug allowed me to upload my capx.