Since firefox-5 it seems that firefox now needs more stuff to be passed. Since you're passing --prefix and --libdir, --target should also be passed. It mainly helps with crosscompilation. Thats why econf is used vs configure.
Reopening until I work out target support.
any news?
Here, have a fix: mozconfig_annotate '' --target="${CTARGET:-${CHOST}}"
Thanks Raul, has been committed to tree.