Try this:

Create 2 new boolean values with these names:

network.protocol-handler.expose.file=FALSE
network.protocol-handler.external.file=TRUE

Open Edit -> Preferences -> Applications. In Content file look for “file”-protocol and change it to use thunar.
(you may need to open a local folder for the “file”-protocol to show in the list. ex. file:///home/user)

tested under firefox 9.