Http Authenticator Component & same contractID ?

[Mfenetre]

Hello,

I would like to write a Firefox plugin handling Http Digest
authentication (401) in a special way for some realms. At the moment,
I've seen that there is :
# an interface : nsIHttpAuthenticator
# a component implementing it : @mozilla.org/network/http-
authenticator;1?scheme=digest

I started to think about how I could buildmy new component. So the
question is : if I want Firefox to use my component to handle Http
Digest authentication, how could I do that ? Do I have to use the same
contractID than the existing one (i.e. "@mozilla.org/network/http-
authenticator;1?scheme=digest") ? But in this case, can I still have
access to the "original" component (buit-in in Firefox) ?

More generally speaking, my question is, how do you "override" an
existing component ? Is it possible to do that ? If I declare a new
component with an already existing contractID, how can I know which
one will be used ?

Thanks in advance for any answers, inputs, ...
Best regards,

Mfenetre