Dê uma olhada nisso. link :
Type about:config into the Location Bar (address bar) and press Enter.
Right-click -> New -> Boolean -> Name: network.protocol-handler.expose.foo -> Value -> false (Replace foo with the protocol you're specifying)
Next time you click a link of protocol-type foo you will be asked which application to open it with.
Once it works, don't forget to reset the value of 'network.protocol-handler.expose.magnet' to True, or FF might keep asking to setup it again.