Vidalia launches successfully, Firefox fails silently.
When launching the TBB using start-tor-browser, Vidalia starts successfully, but 'eats' the output of Tor and Firefox unless --debug is used. Thus the problem that a missing lib needed by Firefox typically results in Firefox silently failing to launch.
As a result of troubleshooting this bug, it was also discovered that when using the --debug option, Vidalia echoes the entire control-port stream, which includes the entire list of relays, which results in 60+ Mb logs. The 'info' debugging level would be much more adequate and useful in most cases, but to get that you would need a (admittedly trivial) edit to the script.