Liveedit protocol handler extension is not compatible with Firefox 4

It is not possible to install the protocol handler plugin in Firefox 4.0b+.

It failed to install "Nuxeo LiveEdit Protocol Handler is incompatible with Minefield 4.0b13pre."

I wonder if providing a mailcap (under Linux) would not solve the problem for other applications as well ?
(see /etc/mailcap on a Linux/Ubuntu system : this is, most of the time, honored by every application)

I don't know how the server detect that the protocol handler is avalaible however and this could still require an extension.