Long running query hogs the server, why?
You need to use Firebird Classic.
Firebird comes in two different architectures, Classic and SuperServer. Super runs one thread per connection, while Classic runs one process per connection. This makes Classic somewhat heavier on resources, but it enables multiple users to run long-running queries and still not create a bottleneck for short transactions.
Since Firebird doesn't have good thread concurency in Super Server, you can use Classic and let your operating system and Firebird lock manager handle things.
Note: Classic was only supported on Linux before, but now there is support for Windows as well.
Alternatively, you can add a second server to do long running reports (OLAP) and keep Super Server for fast transactions (OLTP).