It's true that the toolbar is only officially supported in Firefox 4 and below. However, it's possible to use it in later versions as well, although it does involve a bit of effort.
This article explains the steps: Recover the Google Toolbar in Firefox
In theory, the toolbar is less useful in later verions, because some of its features are now provided within the browser. But I still prefer to use the original toolbar. I am currently using FireFox 14 with the Google toolbar, and it works perfectly.