--- a/build/unix/mozconfig.linux
+++ b/build/unix/mozconfig.linux
@@ -21,8 +21,21 @@ else
   CC="/tools/gcc-4.7.3-0moz1/bin/gcc"
   CXX="/tools/gcc-4.7.3-0moz1/bin/g++"
 fi
 
 ac_add_options --enable-elf-hack
 
 # Avoid dependency on libstdc++ 4.7
 ac_add_options --enable-stdcxx-compat
+
+# $topsrcdir/gtk3 comes from tooltool, when the tooltool manifest contains it.
+if [ -d "$topsrcdir/gtk3" ]; then
+  # PKG_CONFIG_LIBDIR is appropriately overridden in mozconfig.linux32
+  export PKG_CONFIG_LIBDIR=/usr/lib64/pkgconfig:/usr/share/pkgconfig
+  export PKG_CONFIG_SYSROOT_DIR="$topsrcdir/gtk3"
+  export PKG_CONFIG_PATH="$topsrcdir/gtk3/usr/local/lib/pkgconfig"
+  export PATH="$topsrcdir/gtk3/usr/local/bin:${PATH}"
+  # Ensure cairo, gdk-pixbuf, etc. are not taken from the system installed packages.
+  LDFLAGS="-L$topsrcdir/gtk3/usr/local/lib"
+  mk_add_options "export LD_LIBRARY_PATH=$topsrcdir/gtk3/usr/local/lib"
+  ac_add_options --enable-default-toolkit=cairo-gtk3
+fi