The FMOD studio does not recognize old fsb files. I can’t find any download link of the old FMOD designer.
FSB is a ‘destination format’ like .bank is for fmod studio. They are compiled and compressed (a bit like source code compiles into a .exe file). You don’t typically load them.
Do you mean FDP? That is the source format for FMOD Designer. FMOD Studio can load these and convert them into studio format though.
You would need to extract the sounds from an FSB if you wanted to load them into studio. This could be done with a 3rd party tool like fsbextract