While the company hasn't specifically said it's discontinuing it, it does say that it won't work with anything after Firefox 4 and that there will be no more updates.
"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" Google said.
"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" it explained.
Google says that Firefox already has many of the features that the toolbar offered. For those that it doesn't there are alternatives the company added.
Google launched the Google Toolbar a decade ago for Internet Explorer 6 and at the time it added quite a lot of functionality that was missing in the Microsoft browser.
Later when the Firefox version was launched it too enhanced the new Mozilla browser. But now that Firefox is packed full of features it has become redundant.
Then again Internet Explorer 9 comes with a lot of the functionality offered by the Google Toolbar as well but the IE version has just been updated and there's no sign of it getting shut down.
A more likely scenario is that the IE version gets a lot of use while the Firefox version doesn't. This is just speculation but it does make sense. You can pin that down on the different type of users the two browsers have.
While Google hasn't officially updated the Firefox toolbar for newer versions it still works as Google Operating System noticed you just have to install the Add-on Compatibility Reporter extension for Firefox.
This way you can run the toolbar in Firefox 5, Firefox 6, Firefox 7 and so on. Of course with no updates compatibility will eventually break.