None of the reports I've seen have anything to do with crashing. The problems I've been hearing about have to do with print jobs that simply don't print. (They spool, but nothing comes out). Usually, this is associated with HP printer drivers [which have other reported problems with Mac OS 8.5].
There are also issues between FMP 3 (and earlier), andvarious Epson printer drivers (especially the Stylus Color 7xx and 8xx series drivers). FMP 4 doesn't appear to have the same problems. These problems don't have anything to do with Mac OS 8.5.x, as they can also appear in OS 8.1 or even System 7.
Update: Following up on a report by Tommy Luckadoo, Darren now concludes: "FMP 3.0v4 crashed when attempting the second print job using LaserWriter 8.6 on a Mac OS 8.5.1 system. It appears that FMP 3 did indeed have problems with OS 8.5.1. I recommend that anyone trying to use FMP 3 with Mac OS 8.5.x upgrade to FMP 4.1 as soon as possible."
Update: Wally Rodriquez adds: "We experienced printing problems with 3.0v3 FM Pro today and fixed it by turning off desktop printing in MacOS 8.5.1."