From the firefox-13.0.ebuild:
# Both methodjit and tracejit conflict with PaX
mozconfig_use_enable jit methodjit
mozconfig_use_enable jit tracejit
You can quickly check that you are using hardened profile by running:
# eselect profile list
Available profile symlink targets:
 hardened/linux/amd64/no-multilib *
Bear in mind, that using video plugins, flash or java, will very likely crash your browser. An answer to that could be to use Flash/Java in a different web browser, such as Chromium, which requires RWX pages anyway. Alternatively, one could use a browser that cannot benefit from other hardening options during compilation, simply because its source code is not available, for instance - Opera.
If you experience random Firefox or Thunderbird crashes, make sure that you have all of the aforementioned plugins disabled first, and then try again.