From the V1.5.1 Release Notes:

Firebird Superserver has a link-time backward compatibility issue with the NPTL (Native POSIX Thread Library) that may cause it to
be unstable on Linux distributions that enable the NPTL in the GNU C library, e.g. Red Hat 9, Mandrake 10 and Fedora Core. The new
NPTL builds of Superserver should solve these problems.

The downloads/installs are now available. Thanks to Alex for this.

Regards
Paul