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The way we did this works fine for me.
While I would love to use Firefox sandboxed the fact that I don’t doesn’t concern me that much as evidenced by the fact that I don’t recall posting this stuff previously, though I may have mentioned it once months ago.
So this works fine for me on this issue at this time. 
However if the changes were made to Avast to allow for the seamless movement of Firefox back and forth form inside to outside the sandbox it would probably encourage more Firefox users to want to buy an Avast license.