From the nsIContentPolicy Firefox documentation[1]:

Note: The order of content policies in the above case depends on the precedence of installation.

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I recall reading somewhere that chromium used the reverse order, but I'm not certain this is still the case.

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/...

You might have better luck with this on SO but there's no guaranteed order (nor would it make much sense for there to be one)

You could make some very basic extensions, put in some console.logs and find out, but I think it would have to be the order they're installed or random so you can't exploit extensions up or downstream.

There is no specific order. I believe that it is randomized.