Hello Christian Surieux,
Thanks for the URL. I think I've figured it out:
I noticed that you are disabling the Pause and Stop buttons on the client with JavaScript (when a button is found disabled on the page, FormDecorator appends to it a special .rgdDisabled class, and on the other hand FormDecorator does not decorate elements with a CSS class name applied, and the problem is due to the different support for onpropertychange in different browser)s, and it seems that the issue with FF has something to do with the execution of that script.
I suggest that you disable the buttons with a little timeout, in order to ensure that FormDecorator has already done its job on the page. Or another approach - set the buttons disabled from the server, and enable them on the client when the Play button is pressed (or accoring to the logic of the player).
Sincerely yours,
Martin Ivanov
the Telerik team