Thunderbird won't launch a URL in a message

[Douglas Phillipson]

I'm not sure when this started happening but Thunderbird now gives me an 
error when I click a link in a message:

Error showing URL:  There was an error launching the default action 
command associated with this location.

How do I set the "Default action" for a URL?  I don't see anything 
related to a default action in the preferences.

Thanks

Doug P