what should be my next plane
What direction do you want to go? Glow, electric, park fliers? There is a WIDE selection of things to try after a simple plane such as the Firebird.
Most likely you'dd want to go with a trainer type model... 4 channels, glow or electric are available.
Don't try moving directly from the Firebird to a .91 4-stroke powered scale P-51.... it would last about 30 sec. (most large, fast aircraft would give similar results)