firmness, steadfastness, stability, and tenacity
flash, flame, flare, and glare
flat, insipid, tame, and monotonous
fluctuating, hesitating, vacillating, and oscillating
folly, foolishness, imbecility, and fatuity
foolhardy, hasty, adventurous, and reckless
fop, coxcomb, puppy, and jackanapes [jackanapes = conceited person]
force, vigor, power, and energy
formal, precise, stiff, and methodical
fortunate, happy, prosperous, and successful