How to block all Firefox extension installations but one

[Binky]

I administer a PC for a child. To prevent a type of social engineering, I block Firefox extension installation with the following in sandboxie.ini:
ClosedFilePath=*.xpi

Now, however, I would like to allow the auto-attach of the RoboForm adapter extension by allowing access to roboform.xpi within sandboxed Firefox. As I understand, ClosedFilePath takes priority over ReadFilePath in sandboxie.ini (regardless of their order). How do I add an exception for roboform.xpi with the sandbox for firefox.exe?

One way to achieve this is if sandboxie.ini treated conflicting ClosedFilePath and ReadFilePath rules depending on their order. Like firewall rules, the first matching rule would be used.