<<I suspect it is some sort of conflict between libicu linked into fbintl
and libicu used in the macOS system frameworks (and I suspect a change
to icu in macOS is why this only started with 10.13). I tried to rebuild
Firebird with a later version but didn't get very far>>

MacOS does not use a full icu implementation, so I doubt you are linking
against the system icu lib (/usr/lib/libicucore.dylib) unless of course
you are specifically requesting it.

To be honest I doubt you could build Firebird if you hadn't installed
a proper copy of icu from elsewhere.

Personally I expect the problem is elsewhere...
Does it crash on 10.13 or on earlier versions of the OS?
Have you analysed a crash dump?
What happens if you use isql on your embedded version of Firebird?
What version of Firebird are you using?

Paul