Where is the command that is run when 'Open Link' is clicked?

[Chris Green]

Got it!  :-)

The problem is in mimeapps.list (~/config/mimeapps.list).

I guess this *should* have 'exo-web-browser.desktop' for all the file
types that need a browser to display them.  The Preferred Applications
setting will then work.

However my mimeapps.list had firefox.desktop for quite a few of the
file types so setting the browser in Preferred Applications had no
effect on these.  

Replacing all occurrences of firefox.desktop with exo-web-browser.desktop
has fixed my problem.

I *guess* my mimeapps.list file may be quite old and predates the use
of exo for setting preferred applications, as it's a user
configuration a new install won't update it so one is left with
'legacy' settings.

-- 
Chris Green