As whynot hinted, just what the plug looked like B4 being cleaned is critical information for diagnosis.....
(so is a compression check)
As to "guesses", the coil and it's connector pigtail are common issues - followed by plug wires & their connections. "Cracking" of the coil, especially in the base area that can only be seen after removal, is indicative of an issue there. Inspecting the coil connector & it's wiring is always a good idea on these, there's a reason a replacement "pigtail" is available from Ford....
Speaking of "tests", I like to do a quick check for spark with a neon spark tester, either an in-line model or the cheap & quick AC brand pocket pen tester that you only need to hold to the wire to see if you have a good jolt of electricity moving through it!