If playing the Circle of Wildfire Druid, would your wildfire spirit be able to hold an action? For example, if I had them next to me, could I use my bonus action to command them to hold their action to fiery teleport once an enemy gets within range or once an enemy swings on me?

Yeah, I suppose so.

Could that also potentially interrupt an attack? For instance, could they hold their action to teleport once an enemy swings on me, would the teleport activate after they swing or would I teleport away before the swing and they lose their attack?

Quote from Bjbuch6 >>

Could that also potentially interrupt an attack? For instance, could they hold their action to teleport once an enemy swings on me, would the teleport activate after they swing or would I teleport away before the swing and they lose their attack?

1)  Yes, you can order your companion to take the Ready action.

2)  All readied actions take place after their trigger, but it's up to your DM what exactly constitutes a valid trigger.  A trigger of being hit won't let you take an action until after you're hit.  If your DM allows it, a trigger of someone or something looking like it was about to attack would go off before the attack, but the attacker wouldn't lose their attack - they'd be allowed to choose a new target.

Reactions always happen after their trigger unless explicitly specified (such as Shield). If you told it to teleport when someone attacks you - the attack would go off first - and then you would teleport.