So given the choice of Firebolt, Minor Illusion, and for the sake of this thought experiment, Vicious Mockery: Which cantrip should you use and when?
As a reminder, disadvantage will generally be the difference between a hit and a miss, approximately 25% of the time.
Minor Illusion is guaranteed to give an enemy disadvantage on their first attack against a particular ally. You spend your turn to block 1 out of 4 monster attacks.
Vicious Mockery has a chance to give them disadvantage. You spend your turn to have around a 50-60% chance to block 1 out of 4 monster attacks, plus a little damage.
Firebolt just does damage.
So there's a spectrum from most defensive in Minor Illusion, to most offensive in Firebolt. Is defense the best strategy, making Minor Illusion the best? What about as you level up and more enemies get Multiattack? One enemy attack becomes less and less significant, and the comparative damage loss from not casting Firebolt becomes larger and larger. Is your damage contribution, relative to your party, so low that blocking 1/4 of one enemy attack is still worth more than Firebolt's damage?