Variant 1:
- Open prefs.js file from Firefox profile folder
- Find string like user_pref("InFormEnter.1.MenuSet" for first profile, user_pref("InFormEnter.2.MenuSet" for second profile etc
- Parse it here http://firefoxer.ru/informenter+convertor.htm or parse it manually

Variant 2:
- Downgrade Firefox to 56 version
- Install 0.771 version of InFormEnter. It must convert data for Firefox 57. Just in case, export all profiles data (button Export in InFormEnter options)
- Reinstall Firefox 57
- Reinstall InFormEnter 0.841