We don't have malloc_usable_size. Therefore and probably for other reasons too we
need to use jemalloc. Not sure now with enabling (is it needed with the patch below?).
With this we don't have any warnings in about:memory.

ac_add_options --enable-jemalloc
ac_add_options --enable-replace-malloc
diff -Naur mozjemalloc.cpp.old mozjemalloc.cpp
--- firefox-60.5.0/memory/build/mozjemalloc.cpp.old 2019-01-22 18:48:59.477959810 +0000
+++ firefox-60.5.0/memory/build/mozjemalloc.cpp     2019-01-22 18:52:24.655702951 +0000
@@ -1743,7 +1743,7 @@
 #define JEMALLOC_MADV_PURGE MADV_FREE
 #define JEMALLOC_MADV_ZEROS force_zero
 #endif
-  int err = madvise(addr, length, JEMALLOC_MADV_PURGE);
+  int err = posix_madvise(addr, length, JEMALLOC_MADV_PURGE);
   return JEMALLOC_MADV_ZEROS && err == 0;
 #undef JEMALLOC_MADV_PURGE
 #undef JEMALLOC_MADV_ZEROS
@@ -2451,7 +2451,7 @@
         mStats.committed -= npages;
 
 #ifndef MALLOC_DECOMMIT
-        madvise((void*)(uintptr_t(chunk) + (i << gPageSize2Pow)),
+        posix_madvise((void*)(uintptr_t(chunk) + (i << gPageSize2Pow)),
                 (npages << gPageSize2Pow), MADV_FREE);
 #ifdef MALLOC_DOUBLE_PURGE
         madvised = true;