An intermediate flavour-solution
By RAW, Firebolt doesn't produce light by which you could see something.
I DM'd for a group whose wizard had a sneaking suspicion (and he wasn't wrong) that there was something lurking in the high up, dark ceiling of a big cave.
He didn't really have anything to shed light on it, so his reasoning was "I shoot a firebolt up, and the trailing light will reveal what's up there." I told him it doesn't work like that and that his firebolt flashed into the ceiling, harmlessly.
Then he said: "I can pretty much cast a cantrip at will. So I could just fire one after another indefinitely, surely all these "fires" will reveal some form huddled against the ceiling if there was one?"
I allowed him to do it, and made him roll perception (the chances of anyone in the party being able to see the creature was astronomically small). I told him he didn't see anything, after which he entered and promptly got jumped by a giant spider (oh, he's an arachnophobe).
After the session, he protested, saying that he should have seen it and that I was bending the game to force him to be jumped by a spider. I explained that, sure... fire may create some source of light. But it should be seen as the sparks when you try and fail to use a lighter, there was no way he was going to see 100+ feet up in the air in the darkness on a massive ceiling, with a few sparks as his help.
Sure, "flavour" it up with the idea that it creates some momentary source of light, but if you allow it to simply count as a torch or other type of light, you kind of negate "light" spells that are specifically tailored to it. It's also a way for a wizard or sorcerer to simply gear up spells for elemental armageddon, and not having to worry about any utility spells... which is kind of a big deal in non-combat encounters. And the intermissions between combat and non-combat encounters are at the heart of D&D in my opinion.
So if it's in a small dark room and the caster wants to create some sparks to reveal a silhouette, sure, allow it for atmospheric reasons, but don't let it give him any leeway on the proceedings, because that's what "light" spells are for.