Google toolbar is useful and popular ... unfortunately, Google has stopped the development as from the 4th version Firefox, and the latter is recognized as being an incompatible add-on on Firefox 5 or later.
You're disappointed to have lost this valuable tool? No worries, here's a small tip that will help to solve this issue/
Note that: Firefox - You must force the compatibility of add-ons using Compatibility Reporter Add-on . Refer to this FAQ.
As from the 10th version of Firefox
We must first go to the configuration page. To do this, type the following command in the Address bar:
Find this entry "extensions.strictCompatibility" and double click on it to change its value to "True".