Thunderbird URL Won't Work

[Ed Sutherland <digital@twcny.rr.com>]

I'm using Gnome and Thunderbird. At some point today, Thunderbird 
stopped opening the default Web browser when I clicked on a hypertext 
link from within an e-mail. I have to copy the URL from Thunderbird and 
paste it into Firefox.

How do I renable URLs within Thunderbird and Gnome? (It is preventing me 
from using Linux.) Thanks.

Ed