Rated 4 out of 5 stars
I use this addon extensively. It has issues but it works (mostly) for me. Would like to see some development work and some documentation (e.g. why do some fields on some forms not work and how to get them to work).
If you can't get the settings window to display properly (to big), right click on any form field on any page and click "Add a rule for this field..." - this always brings up the settings window correctly. Also, Ctrl+Alt+Shift+J seems to work consistently.
The rules and profiles are stored in prefs.js in your Firefox profile folder. Be sure to back this up regularly to prevent loss of data. Or there is an option (Advanced > Advanced) to "Store form field rules in a custom config directory" so you can put them somewhere that gets backed up regularly. I use this option to share rules between Windows and Linux installations of Firefox.