This - http://www.google.com/firefox - is the old Firefox Start Page used by the Firefox 3.6 and earlier versions of Firefox, I don't think it is being maintained by Google.

Starting with the Firefox 4 version, Firefox is using a "local" Start Page with the address of about:home. It looks similar to the old Start Page, but it isn't exactly the same.

You may have the old start page. Go to Firefox > Options and on the General pane, under startup, click restore to default. The page you probably have (http://google.com/firefox) was only used in 3.6 (and hence tells you you're out of date) but some users seem to have gotten stuck on it after updating to Firefox 4 or higher.

Too bad the Firefox 8.0.1 and 9.0 updates for 3.6.* users did not revert the Google hosted Firefox start page to about:home if it was possible to do without accidentally changing their custom homepage.