Firefox not opening from Thunderbird links: error launching default action command

[R. G. Newbury]

Firefox 3 was hanging on certain websites. I downgraded to Fedora 2.0.17 
. Now when I click on a link in a Thunderbird email I get a box with the 
  error message:

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

Can anyone tell me how/where to fix the 'default action command'?

Geoff