As it turns out, the Ubuntu forums have several threads about Java
website failures when using Firefox 3. There's some discussion on
configuring the Sun runtimes vs. the OpenJDK runtimes, Flash, etc.,
but nothing that really applies to my situation (already using the Sun
runtimes, only one version installed, etc). Some contributers
suggested a fallback to Firefox 2, or installing both 2 & 3.

Since I had this working before, I'm concluding that the update from
Firefox 3 Beta to Firefox 3.0.1 introduced a bug.