Cannot execute /usr/bin/firefox [NB 6.0 M10]

I installed the NetBeans IDE 6.0 M10 on Linux, ran the application,
got the introductory page, clicked on Docs and Support and was treated
to a Warning Dialog stating:

Cannot execute /usr/bin/firefox.  Check external browser configuration.

Well, in the Tools/Options/General dialog box Firefox is selected, and
'/usr/bin/firefox' exists on the file system and is executable.

This is not an auspicious beginning.

How do I fix this problem?