http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60536
--- Comment #15 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Well, even when it is Firefox/whatever bug, the question is why do you get a crash in libgcc_s.so.1. Is that because your libgcc is too old to handle the gcc 4.9 emitted unwind info? Note, I'm not aware of any such changes in the last few years, so it would need to be very old, or -flto generating invalid unwind info or something. Can you make sure you are using libgcc_s.so.1 built by gcc trunk with debug info and see where exactly it crashes?
