"_malloc_prefork" not found

[S.N.Grigoriev]

Hi All,

I've updated from amd64 6.1-RELEASE to 6-STABLE.
All works fine. The only problem: when xmms or
firefox starts the following message appears:

/libexec/ld-elf.so.1: /lib/libpthread.so.2: Undefined symbol "_malloc_prefork"

The Ports tree is fresh. Both xmms and firefox have been rebuilt.
Who knows what have I do to fix the problem?

Thanks,
Serguey.