OK, I now understand about the index size limit of 252 bytes
and that UNICODE_FSS index has a limit of 84 charactes,
assuming no other columns in the index, such as my BIGINT.

I've also downloaded "Release Notes 2.0 - Release Candidate 5"
and I understand that some large portion of this restriction
is/will be elimiated in v2, although it is not yet precisely
clear what the actual limitation will be - just that it will
be "replaced by much-extended limits that depend on page size."

So, thanks to one and all for Firebird and the many resources
available to help continue on with this exciting exploration.