I have this problem as well with some older games (created before September-October 2020 or so) although I have no extensions in Firefox. I was able to fix it by creating a new deck by hand by copying the card faces to my computer and uploading them to my art library. A ton of work. New games don't seem to have this issue for some reason and I was able to transmogrify a deck from a newer game to an older game and the deck worked. I don't have this problem in Chrome, however, so I recommend switching to Chrome.