From MozillaZine Knowledge Base
Want to change where the throbber (or activity indicator) takes you?
Note: Starting in Firefox 2.0, the above method will not work. You will need to install the Throbber Button extension.
You can configure the Mozilla Suite throbber URLs separately for each window. For instance, you could have the Navigator throbber take you to one URL and the Mail & Newsroups throbber take you to a different URL.
Mail & Newsgroups
Note: Until bug 309371 is fixed, the instructions below do not work. The bug report has details of a workaround, but it involves hacking Thunderbird's installed files.
Thunderbird has throbbers displayed in the main window, the Compose window, and the Address Book window. You can change the throbber URL for each separately. For each that you want to change, exit Thunderbird and add the appropriate lines of code below into your user.js file, replacing "http://kb.mozillazine.org/" in the examples below with whatever URL you want to use. You can also change these settings through about:config.
// Change throbber URL in main window user_pref("messenger.throbber.url", "http://kb.mozillazine.org/");
// Change throbber URL in Compose window user_pref("compose.throbber.url", "http://kb.mozillazine.org/");
Address Book window
// Change throbber URL in Address Book window user_pref("addressbook.throbber.url", "http://kb.mozillazine.org/");