I think I've found it with a quick search of the Mozilla repository.
mozilla/accessible/src/atk/Platform.cpp, line 81:
81 #if defined(LINUX) && defined(__x86_64__)
82 libPath.Append(":/usr/lib64:/usr/lib");
If you change line 82 to
libPath.Append(":/usr/lib64:/usr/lib:/usr/lib/x86_64-linux-gnu");
and rebuild Mozilla, it might work.