The fire surface ticks damage twice - once on impact, again at the start of the enemy turn, no saving throw. That's 2-8 more damage on a single target beyond whatever the 1d6 of the initial hit and 1d4 from the burning status do.
As far as I can tell the damage goes as follows: firebolt hits or misses dealing 1d6 or 0 damage, the area under the target is set on fire. The target receives the "Burning" condition, which deal 1d4 damage immediately, and one more time during the targets next turn, after which the "Burning" condition is gone. If the target moves through a fire surface the burning condition is re-applied. If the target does not move and stands still in a fire the "Burning" condition is not applied.
The enemies don't have to move through that burning area. If you managed to snipe 2 goblins standing next to each other - good on you, that feels good to me.
Plus, once we hit level 5 i'd assume the firebolt will deal 2d6 as opposed to 2d10 in 5e, while the burning damage will remain static most likely. But this is conjecture.
That assumption doesn't make sense. Why wouldn't it be 2d6 + 2d4, given that they changed 1d10 to 1d6 + 1d4? You have no grounds for assuming they won't change the Burning or surface damage to scale.
That assumption is based on the Fire Bolt cantrip, which says it deals 1d6 fire damage (at level 1-4), which I would assume is bumped up to 2d6 at level 5, following general 5e cantrip scaling. The 1d4 is not mentioned anywhere in the cantrip, but rather is tied to the "Burning" condition. Which I assume won't scale with levels, because it does not scale in DOS games.
Yes, the fighter may miss, reducing the damage, but so can the firebolt which then will do some mild fire damage.
It shouldn't do any damage at all. That's what missing is supposed to mean. I believe Melf's Acid Arrow is one of the few spells which can do some damage on a miss, and that expends a second-level slot. If you can find some infinite-usage cantrips with attack rolls which can miss and deal damage in the tabletop game that I missed, I'd be glad to hear about them.
My point is that 5e cantrips (apart from Eldritch Blast, which is why it is good) are weaker than multiattack. Because the more attack rolls you make the better. And having a tiny (1-2d4) in case of a miss isn't too much to completely break the game.
