Thunderbird, web links in email, and Firefox

[JD]

Ever since upgrading to F14,
double clicking on a web link in an
email message (in Thunderbird) does
not open that link in Firefox.
My home dir was not altered during
upgrade, as it is on a different drive.
Thunderbird: Edit -> Preferences -> Advanced -> General shows that
"Always check to see if Thunderbird is the default mail client on startup"
is checked.

Any ideas how to restore that functionality?

JD