Hmm, normally cookies are just used for very short bits of data such as your account identifier, and not critical data like lists of words. Are you sure it was in a cookie? Maybe it was in something called local storage or DOM storage, or perhaps it was stored by the Flash player in a Local Storage Object, sometimes called a Flash "supercookie."

Anyway, updating Firefox usually keeps all your cookies. Did the Refresh feature run? The tell-tale sign is a folder named Old Firefox Data on your computer desktop with recently modified files inside. Possibly maybe I'm not sure... you could find your old cookies in there.

If the website was using local storage and not cookies then it is likely that this data is lost when you have used the refresh feature in Firefox.