Although Thunderbird and Firefox share the same Extension Manager architecture, there is one important differenceextension files are installed differently in Thunderbird from the way they are in Firefox. In Firefox, you can click the .xpi file directly and have it installed into the Extension Manager (note that there is no "Install" button in the Firefox Extension Manager, but there is in Thunderbird, as shown in Figure 13-1).
Figure 13-1. Thunderbird Extension Manager.
In Thunderbird, you must right-click the file and save it to a location on your computer, and then install it through the Extension Manager interface (the same procedure applies when it comes to themes). To install an extension in Thunderbird, simply do the following:
After installation, you must restart Thunderbird in order to enable the extension.
Extension Development and Version Numbers
Remember that extensions are constantly under development and may contain different features by the time this book goes to press. Make sure when you download an extension that it is compatible with the version of Thunderbird you are running.
For information on managing, disabling, and uninstalling extensions, refer to Chapter 7.