In FireFox if type about:config in your address bar it brings up a page of browser settings. Ever wonder what those settings are/do? wonder no longer, This page contains an extensive list of About:config entries

Example:

If you go to the following section you can see it tells you about the URL connected to the Throbber. Now if you go to about:config and click on browser.throbber.url you can change the URL the Thrrobber takes you to when clicked.

browser.throbber
Settings for the browser's throbber
 

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browser.throbber.url
Specifies URI to load when browser throbber is clicked