You might have a look at the Libby Overdrive plugin thread. It has instructions for getting a "key" using Firefox or Chrome as part of the importing ACSM process (not the setup suggested in the OP of that thread). Search for firefox or chrome in that thread.
FWIW, I'm using the Libby Overdrive plugin together with DeACSM and DeDRM 10.0.3 to import library books.
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