What I'd do is look at the output of ls -lrt $FWDIR/lib/*.def.
A whole bunch of the files will have similar date/timestamps.
The ones at the bottom of that list may have been modified and may be worth backing up.
If you manage gateways of an earlier version, you may have modified .def files in various backward compatibility packages also.
On gateways, a commonly modified file is $FWDIR/boot/modules/fwkern.conf
In general, anytime you manually modify one of these files, you should document it for future use.