Players create a consistent character from the outset. Rather than using different character models for each class, players simply pick up a specific "Frame" when they spawn. You're essentially picking an outfit off the rack based on how you want to play. Additionally, Red 5 Studios' lead designer Scott Youngblood -- also the designer for the Tribes series -- informed me that each frame is customizable inside its class.
Each frame on display at PAX had three different abilities suited to their class. The medic, for example, had an area-effective healing ability, a supercharge beam and a defibrillator. Those abilities aren't set in stone, however. As players progress in the game and earn experience, new abilities -- and additional ability slots - can be earned. Thus, no class is set in stone, and players will be able to customize each class of frame as they see fit. Furthermore, players can unlock new weapons and modifiers for each weapon's alternate fire mode.
Given the free-to-play business model that FireFall will use, you might guess that Red 5 plans to sell upgrades and weapons via microtransactions, but Youngblood insists that players won't be able to buy their way to victory. He told me that in-game purchases will amount to aesthetic items that don't offer any gameplay advantages.
The final version of FireFall will include more classes and other gameplay modes -- only team deathmatch was on display at PAX East -- as well as the aforementioned unlockable goodies. I didn't have enough time to really suss out the intricacies of the game but, assuming Red 5 delivers on the promise of consistent player progression, FireFall could prove to be an enticing proposition -- especially with a $0 price tag.