Some of the few advantages of using Firebird 3.0.x is that performance was greatly improved vs 2.5.x

Here are a few related performance/tunning articles

In the first article Vlad does quite a few benchmarks between Firebird 2.5/3.0.0/3.0.2 releases . It's very nice that minor releases brings free speedups. He also shows Firebird 3.0.3 performance related changes and fixes and the future ones that are already in Firebird 4
 
 Firebird Performance, by Vlad Khorsun, Firebird Project
 Tuning Linux, Windows, and Firebird for heavy workload by Alex Kovyazin, IBSurgeon.
 Firebird 3: New in Database Statistics and New SQL execution plans, by Dmitry Kuzmenko, IBSurgeon
 Advanced Firebird Trace API, by Vlad Horsun, Firebird Project