Getting external mail program to work under Solaris

I've got mutt working as my external mail program for Firefox PR1 on
my Linux system at home but can't get it to work on my Solaris Sparc
system at work.

I've created a script run-mutt which invokes mutt in a terminal window
(and works fine when called from a command shell).  I've changed the
following in prefs.js (using about:config):-

network.protocol-handler.app.mailto             <full path to run-mutt>
network.protocol-handler.external.mailto        true

... and when I click on mailto: links absolutely nothing happens.  The
run-mutt script doesn't get invoked (I've put some simple output to a
file in there to check).

So have I missed anything or is there something broken about Firefox
PR1 on Solaris?

By the way what does the network.protocol-handler.expose.mailto
parameter do?  Some places say to change it to TRUE to run an external
program and other places don't mention it at all.  (I've tried setting
it but it made no difference)

-- 
Chris Green