There's a reason for even database servers with 32-bit transaction IDs to be 64-bit processes - performance and memory. AFAIK until Firebird 3.0, Firebird also

There's a reason for even database servers with 32-bit transaction IDs to be 64-bit processes - performance and memory. AFAIK until Firebird 3.0, Firebird also had 64-bit server builds with 32-bit transaction IDs - it would have broken many external tools had the old on-disk format been changed.