What are those NPTL packages?
In Linux kernel 2.6, a new threading library has been introduced. It is known as Native POSIX Thread Library or NPTL. This library enables programs that use native POSIX threads to run very effectively. For example, it takes NTPL program about two seconds to start 100.000 threads, while non-NPTL kernel would need around 15 minutes for it.
NTPL builds only exist for SuperServer as it uses threads for each connection (Classic uses a separate process for each connection). You should download and install the appropriate Firebird package for your system: NPTL package on NPTL system and non-NPTL on non-NPTL.
If you are unsure whether your system has NPTL support, you can check with:
If you have NPTL support, you should get something like: NPTL 2.3.5.
If you don't have NPTL support, you should get something like:
getconf: Unrecognized variable `GNU_LIBPTHREAD_VERSION'
or some other output that does not contain NPTL version.
Make sure you don't install the wrong version as it can cause malfunction of Firebird - slow connections, sporadic failure to attach the database, etc.