http://lgms.nl/p/factorio/fff-search.html
An example would be if you want to know what this sprite atlas option does, which was explained in FFFs:
http://lgms.nl/p/factorio/fff-search.html#atlas
Let me know if you think it's useful, then I'll make sure it's updated with the latest FFFs

Why not use duckduckgo with "site:factorio.com"? I don't know, this feels nicer for some reason. Maybe it's because this search is so fast, you can try five queries until you find the right keyword, without having to wait for results to load?