Can't get Firefox to act as default browser

[J.Pilk at tesco.net (John Pilkington)]

Hi: fc10 x86_64 running kde 4.3 from updates.

I used to have Firefox as default browser.  Now web links from 
Thunderbird always get Konq.

Within Firefox, Preferences > Advanced > System Defaults says that 
Firefox is already my default browser.

Settings > Preferred Applications > Web Browser can be set to Firefox, 
but gets reset to 'Custom' with all the options greyed out and the 
Command shown as  '/usr/lib64/firefox-3.0.13/firefox "%s"'

I haven't tried the script given here

http://userbase.kde.org/Tutorials/Modify_KDE_defaults

which seems to be intended for cases in which the System Settings route 
is inconvenient and looks like overkill for my case as a sole user.

Any suggestions appreciated.

John P