I have found the required registry entry for the java plugin 1.4.2 to work with the current version of MozillaFirebird. Somewhere in NPOJI610.dll it checks for this:
[HKEY_LOCAL_MACHINE\SOFTWARE\mozilla.org\Mozilla]
"CurrentVersion"="1.0"
The version value can be anything but if you don't have this registry entry the java plugin won't start.
Minh Tran-Le.