I think you misunderstood -- it's not a goal but a measurement.

The actual statement, if you read the original source[1], is

>Firefox 7 uses less memory than Firefox 6 (and 5 and 4): often 20% to 30% less, and sometimes as much as 50% less.

[1] http://blog.mozilla.com/nnethercote/2011/08/09/firefox-7-is-...