The home button in the Firefox web browser – and actually in every web browser – opens the default homepage set in the web browser when it is clicked.
Most users do not need that functionality because of better alternatives that are provided by browsers. It is for example possible to add the homepage to the visible bookmarks, use a keyboard shortcut to open that website or to enter part of it in the address bar and load it then instead.
While some may be used to using the home button to open their homepage, most users probably ignore that button completely.
The developer of Usable Home Button for Firefox must have had similar thoughts. The add-on changes the standard behavior of the home button in Firefox so that it always leads to the root level of the active website.
So, if you are on a page like http://www.ghacks.net/2009/11/28/finally-some-good-news-for-the-firefox-home-button/ it will take you to http://www.ghacks.net/ automatically when you click on the button.
This can be quite handy as it is not always a given that a webmaster included a link back to the root page of the website on the page you are on. While you can modify the address manually by removing everything but the root domain, clicking the home button instead is definitely faster.
Firefox users who are interested in this functionality can download the Usable Home Button add-on for Firefox at the official Firefox website.
The add-on is compatible with all versions of the Firefox web browser including the very latest versions.
The most recent version offers another feature of interest. When you right-click on the home button, all directory levels of the site become accessible right from a context menu as well.
Besides that, you can also use it to make Firefox forget about the page or site (which will delete it or the website from the browsing history), to unload the site and display an empty tab page instead, or to load the browser’s start page (the home page configured in the browser).