This button calls up the Mozilla Sunbird calendar from Firefox - to version 3.* - and Thunderbird - to version 3.*. This version is intended for Windows 64-bit operating systems, principally Windows Vista and 7.
It depends upon having the current version of the Mozilla Sunbird calendar downloaded from the distribution site and installed on your computer.
First, download and install the current Windows version of Sunbird from http://www.mozilla.org/projects/calendar/sunbird/
(Skip this if you already have Sunbird installed.)
Then download and install the add-on in FF/Thunderbird as normal. Then use "Toolbar-Customize" to locate the icon and then drag it to the toolbar (or sidebar), to wherever you would prefer it..
Sunbird logo copyright of and used with kind permission of Mozilla Foundation