Thanks FullOppositeRudder. I had a quick look at that and the default location for ReminderFox data is the Firefox profile folder. So locating and copying that should work.
(On a slightly related note: I moved a Thunderbird installation to a new computer just yesterday. Copied the profile folder across and pointed the default profile for a new installation to this folder. Instant result: everything back where it was, as on the old system.)