The Thunder has a "hooked" triggerguard, the Firestorm is rounded. The Firestorm has wraparound grips with finger grooves, the Thunder has grip panels and a smooth front on the grip frame.
There are slightly different options between the two as far as finishes - one can be had with chrome frame/blue slide, the other can be had with blue slide/chrome frame, etc.
Other than that they are the same gun with different monikers stamped on them. I believe the Firestorm name is on guns made for the importer (Eagle Imports?) and the Thunder name is used by Bersa. Why they did this is anyone's guess because they (Eagle) import both guns - and Bersa manufactures both!
Hope I explained it so it makes sense to you!
Underrated guns in my opinion. I have a Thunder .22 that's been 100% reliable with CCI Blazers for the 7 or 8 months I've owned it. Sure is a fun little gun to plink with.