Google Toolbar for Firefox extends the browser even more allowing you to share pages directly, write side-wikis about the pages you are viewing, auto-translate pages instantly in Firefox and much more.
Other features include a multi-language spell check, Google search box and a built-in auto-filler for forms and other fields. You can even check your Gmail via the toolbar without having to open your inbox within Firefox. Other add-ons you may choose to select or deselect is a pagerank tool. The number of buttons you can add is a bit overwhelming in fact and can soon leave Firefox looking bloated and cluttered. You can also sync toolbars across two computers so that they are identical on both machines.
Most worryingly however is the memory leak associated with Google Toolbar. I was using the latest version of Firefox (version 3.6.3) on Mac and found that at random moments, it leaks memory terribly meaning that Firefox simply freezes with the spinning wheel of death crashing the browser. I haven't experienced the same problem on Windows although it is listed by Mozilla as an add-on that is susceptible to this problem. The only solution is to deactivate or uninstall it in your Add-Ons menu and wait until Mozilla and Google find a solution.
Google Toolbar is designed to increase productivity and make life easier although if you experience memory leaks with the latest version of Firefox using it may be more trouble than it's worth.
Translates pages automatically in Firefox
Allows you to share web pages instantly
Write your own side wiki opinions of web pages
Lots of other browser enhancements
Can get cluttered with too many gadgets
May cause memory leak problems
"Downloading & installing Google Toolbar for Firefox"
Firefox refuses to allow the download as it states that this version is not compatible with Firefox 5.