All I know is is that FWR damages the internal components of Simplex-brand alarms over time. All other manufacturers have included FWR-protection circuitry in their alarms by default, but not Simplex for some reason (even so most alarms will sound pretty bad on FWR, like Spectralert Classics).
I have a 4903-9236 that I had to replace the strobe on thanks to unknowingly powering it with FWR, & now I’ve discovered that I might as well replace the horn too, because the FWR seems to have raised the pitch of the horn somehow (and thus it doesn’t sound like a 4903-series electronic horn/strobe should).
Here’s a topic discussing the issue: FWR and DC - #5 by firealarm2905