Yes, when it is used to target a creature
Sorcerer's Twinned Spell Metamagic feature reads:
When you cast a spell that targets only one creature and doesn't have a range of self, you can spend a number of sorcery points equal to the spell's level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).
To be eligible for Twinned Spell, a spell must be incapable of targeting more than one creature at the spell's current level.
The meaning of the first sentence might be more clearly understood as a precondition, not a limitation. That is, before Twinned Spell becomes an option for the caster, the sorcerer must first cast a spell that targets a single creature. Once that condition has been met, we can use Twinned Spell, provided that the spell can't target more than one creature.
Many spells, as you point out, may target either creatures or objects. The point of Twinned spell is not to disallow these spells from being used at all. It is simply limits Twinned Spell to those situations when the spell is being used to target creatures.
Put differently, with spells that may target creatures and objects, the sorcerer is free to do either. She may only twin the spell if she elects to target a creature (and the spell is unable to target more than that one creature).
In your particular case, Firebolt is only ever a single bolt (unlike, say, Eldritch Blast). So it is eligible to be twinned provided you are using it to target a creature.