Yahoo! has released [Yahoo! Toolbar for Firefox](http://toolbar.yahoo.com). Some previous reports said that the toolbar is [only available for Windows](http://www.internetnews.com/xSP/article.php/3482141), with MacOSX and Linux version coming soon. But, I was able to install the toolbar under Linux without any problem.
The screenshot was taken under Linux to prove the toolbar works under Linux as it should :).