Google Toolbar for Firefox has long been a ubiquitous add-on for the popular open source browser, but Google decided it's no longer needed.
"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 browser. 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," says a message on the Google Toolbar help blog.
Google has been cleaning house recently, killing off projects and services, most notably Google Labs.
Since Google has a browser of its own, it's no wonder that Firefox is not its first priority. However, judging from the comments on Google's blog post, many users aren't too happy with this change.