Spells that can target more than one creature can't be twinned, even if they are used to target only one.
Spells targeting objects can't be twinned, and Fire Bolt can target either creatures or objects.
Are both conditions for twinning -"single target" and "creature target"- applied based on what a spell is capable of ?
Can Fire Bolt be twinned when used on creatures?
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.
As user wumpus7 points out the Sage Advice Compendium has been updated since this answer was first posted and now contains more detail about what kinds of spells are legal options for the Twinned Spell Metamagic feature.
Specifically, the designers are explicit stating that the intent of the rule is to preclude the use of any spell which can target an object. Curiously, the wording they use leaves the door wide open for a DM to disagree and rule otherwise:
consult with your DM, who has the final say. If the two of you are curious about our design intent, here is the list of things that disqualify a spell for us:
- The spell can target an object.
I think the choice of wording here is unusual for its subjunctive tone. That is, rather than talking in absolutes, the designers say this is how the feature works "for us."
A table of players is always free to alter the rules in any way(s) they see fit so it's not like the wording in the Sage Advice Compendium is offering license where none existed before. I think it just serves to highlight how much gray area exists around the feature.
From the Sage Advice Compendium (Sorcerer section):
Can my sorcerer use Twinned Spell to affect a particular spell?
You can use Twinned Spell on a spell that …
- targets only one creature
- doesn’t have a range of self
- is incapable of targeting more than one creature at the spell’s current level
If you know this rule yet are still unsure whether a particular spell qualifies for Twinned Spell, consult with your DM, who has the final say. If the two of you are curious about our design intent, here is the list of things that disqualify a spell for us:
- The spell has a range of self.
- The spell can target an object.
- The spell allows you to choose more than one creature to be affected by it, particularly at the level you’re casting the spell. Some spells increase their number of potential targets when you cast them at a higher level.
- The spell can force more than one creature to make a saving throw before the spell’s duration expires.
- The spell lets you make a roll of any kind that can affect more than one creature before the spell’s duration expires
Fire Bolt can target an object.
Thus, unfortunately, the answer is no.
