Drive yourself or Shuttle
Bonnaroo offers a shuttle from Nashville or there is bus service from Murfreesboro if you purchase a hotel package.
This is written from the perspective that you aren't going to be camping on site. If you are camping you can simply walk to the festival. Firefly has no bike parking onsite, so biking is surprisingly and unfortunately not an option at Firefly. In rare circumstances you may be close enough to walk, which is a great option. More than likely you will need to get to Firefly by car. You can Uber to Firefly, and honestly it isn't as bad as other festivals simply because there is not the huge rush to leave at the end of the night because everyone is camping. That said, it will still be cheaper to drive yourself as there is ample parking and it is free to park onsite. I generally go with Uber as we don't have a designated driver, but if you do than driving is a piece of cake as there is very little traffic given everyone is camping.
If you go with the Nashville Shuttle it will be $200+ per person.
Yes, Bonnaroo offers parking on site but you will need to snag an additional parking pass with your ticket.