FFScouter

Visit FFScouter and register a LIMITED API key with the website.

Created by Glasnost [1844049]

Then download this handy tool: FF Scouter V2

Created by Weaver [4157224] and Xentac [3354782]

In Tampermonkey, open the script menu and enter your API key when prompted. Once set, visiting any profile will show:
Fairfight rating + estimated stats under their username.

This is extremely useful during wars and general gameplay for judging opponents.

FFScouter also offers a Premium version with expanded faction and player features.