Best bar/lounge/club I've been to in Pittsburgh. Small intimate bar area on the first floor, a large bar area/dance floor on the second floor, and a great deck. As some of the other reviewers have mentioned, the music is different from any other place in Pittsburgh - downtempo, electronica, etc. Firehouse Lounge has the most "european" feel of any club I've been to in Pittsburgh, and that's a good thing.
The bartenders are quick, drink prices are reasonable (this is Pittsburgh, after all), and the atmosphere is amazing. The crowd seems to be a mix of young professionals/students in their 20s-30s and is diverse and international.
I will definitely be going back.