Building Firefox with:
make -f client.mk profiledbuild

Results in:
...
/var/tmp/moz-build-dir/dist/bin/xpcshell: error while loading shared libraries: libxul.so: cannot map zero-fill pages: Cannot allocate memory
make[4]: *** [prepare-package] Error 127

 % readelf -lSW ./libxul.so
There are 41 section headers, starting at offset 0x103e7f10:
Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
...
  [28] .bss              NOBITS          000000000bfaabe0 bfa9be0 80186ae8c 00  WA  0   0 32

Adding Wl,-Map,libxul.map when linking libxul shows:
...
 .bss..LPBX1    0x000000000ccb3be0 0x800000000 /var/tmp/moz-build-dir/toolkit/library/../components/protobuf/once.i_o
....

Reducing once.cpp leads to:

 % cat test.ii
class A
{
    virtual void Run ();
};
class B:public A
{
public:
    B ();
    void Run ()
    {
    }
};
void *operator     new (unsigned long)
__attribute__ ((__externally_visible__));
void *moz_xmalloc ();
inline void *operator
new (unsigned long)
{
    return moz_xmalloc ();
}

inline A *
NewCallback ()
{
    return new B;
}

 % c++ -c -fprofile-generate -fdata-sections -O2 test.ii
 % readelf -lSW ./test.o | grep "800000000"
  [11] .bss..LPBX1       NOBITS          0000000000000000 000120 800000000 00  WA  0   0 32