Ok now I can say for sure it is the new flash plugin fault, not firefox. I uninstalled it and everything is ok now.
** Package changed: unity-2d (Ubuntu) => flashplugin-nonfree (Ubuntu) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/783904 Title: firefox crashes unity-2d -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs