Firefly, a startup that allows rideshare drivers to make money through digital advertising, is officially launching today. It’s also announced that it has raised $21.5 million in seed funding. The idea of sticking advertising on a cab isn’t new, but Firefly offers drivers what it calls a “digital smart screen,” allowing advertisers to run targeted, geofenced campaigns. The company has apparently run more than 50 ad campaigns already, during a beta testing period in San Francisco and Los Angeles, with hundreds of cars on the road. “Being the first at building out the IP is going to be the main differentiator,” said co-founder and CEO Kaan Gunay.