The 25 FV Cash bonus is avialable by clicking the link in the sidebar at http://farmville.com
On the left there is an announcement about the toolbar. It doesn't matter if you already installed it. You can do it again.
Sometimes at the end of the process you get to farmville and it says there is 25 cash in the giftbox but it is not there. However if you open the page again or restart the browser, the 25 cash really do appear in the gift box.
It is available for Firefox 2.0 and supposedly for IE7 if anyone still uses that. It doesn't work in Chrome or IE8 or Safari it seems.