Depends on the offense. 
It depends on location.
Typically employers have an obligation to pay for return travel if it’s an expense they would have incurred should you had stayed employed. So if they are going to fly you somewhere and back, and fired you while there they’d still pay for the return flight.
If you are a roady traveling with a band, they weren’t flying you home either way so you best try your luck hitchhiking.
It’s generally good policy to make sure former employees can get home, because bitter employees are likely to sue as a result of being stranded. There is a good chance they’ll win if it turns out you’re just being an asshole.
Comey did return on a private jet. I read that the tag numbers on the jet correspond with a jet leased by the federal government. It’s likely they paid to fly him home.
I am being told that some football man named Lane Kiffin got stranded at an airport upon being fired from his footballing job.
this year a NHL team fired their coach right after a road game (which his team lost). The team flew on to their next game on the road and he took a cab back to a local hotel. He flew home the next day and they paid for his flight home as well as the hotel charge.
I had a disagreement with an employee 10 years ago while on a business trip in Chicago and I ended the conversation by telling him to do whatever he wants, but it’s a long walk back to Boston. In hindsight it probably wasn’t the best way to handle the situation, but in my defense I would have called HR before doing anything official.
Missed the edit window, but I checked it out later and we would have reimbursed him for his return trip.
More correctly I’m assuming the director of the FBI doesn’t travel alone, he would have had assistants and deputies who were still employed by the FBI and still needed to go back to Washington. So the jet was going back to Washington anyway, they didn’t pay anything extra for Comey to be on there.