Ok, made it with an oldfashioned batch file for now. Not the most elegant way, a command window will open 'til Mozilla starts. It seems to work, at least for me. Only tested with Firebird, but should be the same for Mozilla.
To add the "Open in Mozilla" option to the right click menu of internet shortcuts:
First copy this text into notepad, adjust the part in red to the actual name and path of your mozilla executable. Save it as "moz.bat" to a folder.
Make sure that it is only one line. No breaks.
for /f "usebackq tokens=2 delims== eol=B skip=1" %%a in (`find "http" %1`) do start /b /d "C:\Program Files\mozillafirebird" mozillafirebird.exe -url %%a
Step two, copy the text below into notepad, adjust the part in red to the path where you saved the batch file. Save it as "Moz.reg" then right click - merge.
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\InternetShortcut\shell\mozilla]
@="Open in Mozilla"
[HKEY_CLASSES_ROOT\InternetShortcut\shell\mozilla\command]
@="\"C:\\folder\\moz.bat\" \"%1\""
-------------------------------------------------------------------------------
To undo the whole thing again merge this last regfile and delete the batch file.
Windows Registry Editor Version 5.00
[-HKEY_CLASSES_ROOT\InternetShortcut\shell\mozilla]
.