Yahoo! hasn't updated their Toolbar extension yet. An update should be available in a few days.

Keep using Firefox 5, it has better security. The Yahoo! Toolbar should still work just fine with Firefox 5, but until Yahoo! releases a toolbar compatible with Firefox 5, you need to disable add-on compatible versions verification. You can do this by installing Mozilla's Add-On Compatibility Reporter extension from https://addons.mozilla.org/en-US/firefox/addon/15003 and restarting Firefox twice. It basically tells Firefox to ignore the maximum compatible version so the toolbar can be enabled on version 5 too, not blocked just because Firefox releases major version numbers like mad just for the sake of catching up with IE9 and Chrome 12.