www/firefox and mail/thunderbird fail to compile in FreeBSD 10-CUR/amd64 with CLANG

On my FreeBSD 10 boxes, all compiled with CLANG and using CLANG (
FreeBSD 10.0-CURRENT #0 r234500: Fri Apr 20 21:59:02 CEST 2012),
compiling/updating Firefox to V12 and Thunderbird to V12 fails with the
below shown error.

Does someone have any clue what could trigger the problem?

On FreeBSD 9-STABLE/amd64, also compiled with CLANG, there is no such
problem.


Thanks in advance,

Oliver


[...]
In file included from
/usr/ports/www/firefox/work/mozilla-release/js/src/jsalloc.cpp:40:
In file included from ./jscntxt.h:50:
./jsapi.h:2102:1: error: 'JS_GetNaNValue' has C-linkage specified, but
returns user-defined type 'jsval' (aka 'JS::Value') which is
incompatible with C
      [-Werror,-Wreturn-type-c-linkage]
JS_GetNaNValue(JSContext *cx);
^
./jsapi.h:2105:1: error: 'JS_GetNegativeInfinityValue' has C-linkage
specified, but returns user-defined type 'jsval' (aka 'JS::Value') which is
      incompatible with C [-Werror,-Wreturn-type-c-linkage]
JS_GetNegativeInfinityValue(JSContext *cx);
^
./jsapi.h:2108:1: error: 'JS_GetPositiveInfinityValue' has C-linkage
specified, but returns user-defined type 'jsval' (aka 'JS::Value') which is
      incompatible with C [-Werror,-Wreturn-type-c-linkage]
JS_GetPositiveInfinityValue(JSContext *cx);
^
./jsapi.h:2111:1: error: 'JS_GetEmptyStringValue' has C-linkage
specified, but returns user-defined type 'jsval' (aka 'JS::Value') which
is incompatible
      with C [-Werror,-Wreturn-type-c-linkage]
JS_GetEmptyStringValue(JSContext *cx);
^
./jsapi.h:2819:1: error: 'JS_ComputeThis' has C-linkage specified, but
returns user-defined type 'jsval' (aka 'JS::Value') which is
incompatible with C
      [-Werror,-Wreturn-type-c-linkage]
JS_ComputeThis(JSContext *cx, jsval *vp);
^
In file included from
/usr/ports/www/firefox/work/mozilla-release/js/src/jsanalyze.cpp:40:
In file included from ./jsanalyze.h:44:
In file included from ./jscompartment.h:46:
In file included from ./jscntxt.h:50:
./jsapi.h:2102:1: error: 'JS_GetNaNValue' has C-linkage specified, but
returns user-defined type 'jsval' (aka 'JS::Value') which is
incompatible with C
      [-Werror,-Wreturn-type-c-linkage]
JS_GetNaNValue(JSContext *cx);
^
./jsapi.h:2105:1: error: 'JS_GetNegativeInfinityValue' has C-linkage
specified, but returns user-defined type 'jsval' (aka 'JS::Value') which is
      incompatible with C [-Werror,-Wreturn-type-c-linkage]