Older (pre WebExtensions) versions of Form History Control stored extra information in the cleanup.sqlite file. This file is located in the root of your profile folder, you can simply delete this file.

For an explanation on where to find the profile folder see: https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data.

I do provide an export/import utility, see: https://github.com/stephanmahieu/formhistory-export