Hm, something interesting happened...
I rebuilt my system (release, not STABLE or even CURRENT) (yes, I'm tinkering a lot
), and now I don't get the error I used to get...
At least right now firefox is building (and even with both CPUs, although I didn't explicity ask for that).
BTW: does anyone know how to change the title to indicate this issue has been solved (at least kind of)?