PIX 6.3(5) was released Aug 12 2005, and is available from the usual cisco locations.
PIX 6.3(5) appears to be a bug-fix release; a mix of problems, with SIP related issues slightly more prominent. There are, as usual, still unresolved problems; one that might be of particular interest is,
CSCeg13789 PIX - compiled ACLs become corrupted over time
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