Firebird 1.5 rc5 released rc6 is on the way
Fixed some server crashes (related to plan genaration and any combination of subselect & between )
There was a bug in log(x,y) function in fact it returned log(y,x), another one was about incorrect nested aggregates tracking

Full Release notes
http://sourceforge.net/project/shownotes.php?release_id=176032
Files for Linux (CS only -no Super Server yet)
http://prdownloads.sourceforge.net/firebird/FirebirdCS-1.5.0.3773-RC5.i686.rpm?download
http://prdownloads.sourceforge.net/firebird/FirebirdCS-1.5.0.3773-RC5.i686.tar.gz?download

The Windows kit should be unzipped over the top of an existing RC4 installation.
You need a rc4 setup
http://prdownloads.sourceforge.net/firebird/Firebird-1.5.0.3744-RC4-Win32.exe?download
and then
RC5 Files for Windows (Zip only - no installer yet)
http://prdownloads.sourceforge.net/firebird/Firebird-1.5.0.3773_RC5_win32.zip?download