component/firefox: use .format string subsitutions

so that we don't have to escape % as %%

component/firefox/buildout.cfg

@@ -37,11 +37,11 @@ script =
   part = self.buildout[os.path.basename(firefox)]
   with open(%(location)r, 'w') as f:
     f.write("""#!/bin/sh -e
-  cd %%s
-  export LD_LIBRARY_PATH=$PWD:%%s
-  export PATH=%%s:$PATH
+  cd {}
+  export LD_LIBRARY_PATH=$PWD:{}
+  export PATH={}:$PATH
   exec ./firefox "$@" 2>/dev/null
-  """ %% (
+  """.format(
     firefox,
     ':'.join(part['library'].split()),
     ':'.join(part['path'].split()),