Google toolbar is a great added value to Firefox and you realize it even more when it's gone!
The version 7.1 is now reported as incompatible by Firefox 5.0, and why would Google to rush to publish a compatible update if this can bring more frustrated users to Chrome ?
If, like me, you're (about to but) not just ready yet to dump Firefox for good, here's the simple and quick fix to force Google Toolbar 7.1 to load in FF 5:
1. In the url bar, type: "about:config" and then click the warning button.
2. right click on the page and select New > Boolean3. As the string, type: extensions.checkCompatibility.5.0
4. As value, select False
5. Restart Firefox
You should now see the Google toolbar enabled in the Add-ons page with the warning confirming that Add-on compatibility checking is disabled.
The Google toolbar 7.1 does work fine, but you may need to disable manually any other incompatible extensions to keep your browser stable (in particular if you get an error such as "TypeError: Components.classes[cid] is undefined").
Alternatively, you can install an add-ons like "Compatibility Reporter" which also turns off the compatibility checking.
And for Firefox 6, ...
extensions.checkCompatibility.6.0...
Since I posted this article, Google officially announced the end the Google Toolbar, so there's no update to expect for current and future versions of Firefox. Also, from FF version 6.0.2, login from the toolbar stopped working. Time to learn to do without it I guess...