firefox build problem

[Gary Aitken]

just did a ports update, haven't done one for a few months.

portmaster -w www/firefox
craps out with:

In file included from
/usr/ports/www/firefox/work/mozilla-release/gfx/2d/Blur.cp
p:7:
In file included from ../../dist/include/mozilla/gfx/Blur.h:12:
../../dist/include/mozilla/CheckedInt.h:179:31: error: non-type template
      argument of type 'const bool' is not an integral constant expression
    static const bool value = IntegerType(-1) <= IntegerType(0);
                              ^
../../dist/include/mozilla/CheckedInt.h:343:28: note: while checking a
default
      template argument used here
  return IsInRangeImpl<T, U>::run(x);

Have I got something screwed up?
I don't know squat about c++ templates and can't see what's wrong.

hints?