heard of the before. love them. sound most like a symphonic/power metal group.
Freedom Call (they utilize the multiple harmony singers very very well, which Firewind seems to use quite a bit).
Angel Dust (their progressive stuff, not the thrash metal they started as).
Some of Firewind's songs have a sort of epic and opera sound to them, which reminds me of Kamelot and Domine. Unfortunately Kamelot just lost their lead singer. Sadly, Domine doesn't seem to be that well known but is a great Italian power metal band.
Listened to some other Firewind stuff. They seem to like synthesizers, which definitely reminds me of Dream Theater.
Also thought of Epica, Nightwish (before they got rid of Tarja), Primal Fear, Gamma Ray, Balance of Power, and Helloween.
I am drawing a blank on others that sound similar. Don't have my music collection to look at at the moment. At the wrong computer.
These are a good starting place i think.