Google is going bullish on their newly launched Google Plus service. As a result, they're making lot of changes to their existing products. Once such product, Google Toolbar for Firefox is being discontinued for the latest versions of Firefox (5 and higher). Here's what Google says in their post: Google Toolbar Help: An update on Google Toolbar for Firefox -

First of all, we'd like to thank all of our loyal users of Google Toolbar for Firefox. We deeply appreciate all of the feedback over the years that helped to make the product so useful. As we all know, over the past few years, there has been a tremendous amount of innovation in the browser space. For Firefox users, many features that were once offered by Google Toolbar for Firefox are now already built right into the browswer. Therefore, while Google Toolbar for Firefox works on versions up to and including Firefox 4 only, it will not be supported on Firefox 5 and future versions. Please see our #-Link-Snipped-# for additional details.