Extensions are designed for Firefox, but sometimes you can use them elsewhere.
Individuals might move nimbly to Firefox, but organizations can take far longer. Some folks out there are still trying to escape Netscape 4.0 or Internet Explorer 5.0. A typical slow upgrade path is Netscape 4.x→Mozilla Application Suite 1.x (classic theme)→Firefox. You don't have to wait for Firefox to start benefiting from extensions, though. A significant number of extensions also work with the Mozilla Application Suite (MAS) and possibly in other tools as well.
In principle, you can design an extension [Hack #84] to install on all Mozilla-based applications. In practice, the extra development effort required to provide portability discourages most extension creators.
Thunderbird and NVu use the same extension-management system as Firefox. Because they are not browser products and because extensions typically integrate closely with Firefox's XUL interface, most extensions are unlikely to be portable. Extensions that hang off the Firefox Tools menu and extensions that act as independent mini-applications (such as calculators, games, and accessories) have some chance of successful integration. They might possibly hang off the Thunderbird Tasks menu.
The first step required for that integration is to hack the install.rdf file that's supplied in the extensions installable .xpi file. In ...