It turns out that Google no longer wants to update Google Toolbar for Firefox, but it doesn't admit that the product has been discontinued.
"Google Toolbar for Firefox is compatible with Firefox version 4 or older. If you use Firefox version 5 or newer, you won't be able to use Google Toolbar."
Google suggests a long list of add-ons that could replace Google Toolbar's features, but the suggestions are too generic. For example, Google links to the search results for [bookmarks sync] or [language translate] in the Firefox add-ons gallery.
A Google blog post offers an explanation: "many features that were once offered by Google Toolbar for Firefox are now already built right into the browser" and thanks the loyal users. That's also true for the IE toolbar, but there are many useful features that aren't included in the browser: auto-translation (a built-in Chrome feature), Google Bookmarks integration, using Google Docs to open documents, smart spell-checking using an online service, highlighting search terms, suggestions for navigation errors (another built-in Chrome feature), custom buttons and gadgets.
You probably remember that Google Toolbar for Firefox was released in 2005, five years after the Internet Explorer version. At that time, Firefox users who wanted to install a Google Toolbar with PageRank support could try an unofficial extension called Googlebar. Maybe that extension will be resurrected, now that Google Toolbar for Firefox is no longer available. Releasing some of the source code under an open-source license would be helpful.
For now, Google Toolbar still works in the latest Firefox releases if you install the Add-on Compatibility Reporter first and restart the browser. Here's Google Toolbar in Firefox 7 Alpha 2 (Aurora):
Last year, Microsoft's Bing Bar dropped support for Firefox and became an Internet Explorer-only add-on, just like Google Toolbar. Bing Bar is powered by Silverlight, a software for running rich internet applications. A few months ago, Google tested a new Google Toolbar powered by Chrome, but only for Internet Explorer. It's likely that the new toolbar didn't work well in Firefox, so Google decided to only offer an Internet Explorer version.
{ Thanks, Colar. }