Google homepage showed for a brief period a link to Firefox 2.0 "optimized for Google", that is bundled with Google Toolbar. It's not the first time when Google recommends Firefox on the homepage (here's another screenshot), but this time Google is more pushy. Also, last time Google explicitly stated that its version of Firefox included Google Toolbar - now you can find out this little detail if you click on the big "Get it now!".
Yahoo promotes Internet Explorer 7 on each and every search results page and promises you can "get Yahoo! Search results faster". But while Yahoo targets IE6 users that will upgrade to IE7 anyway, Google wants to convert Internet Explorer users to Firefox, but also targets Firefox users that don't have Firefox 2.0.