Not sure what to tell you. It works fine in the full Mozilla package. Have you reinstalled the JRE and made sure the Control Panel applet is pointing to the right place?
Open Source is free like a puppy is free.
It's only when you look at an ant through a magnifying glass on a sunny day that you realise how often they burst into flames.
download http://plugindoc.mozdev.org/resources/firebird.reg and run it (you will need to edit the file if your Firebird executable is not at C:\\program files\\MozillaFirebird\\MozillaFirebird.exe, leave the double slashes) then install Java
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