Install Vana'diel Clock Extension Version 1.0.4
This page is due to a bug in the way Mozilla and Firefox handle extensions (namely, they will refuse to install an extension if the page is still "loading", fail to flag an error, and refuse to install the extension until you restart the browser). The idea is that this page is very small and will allow you to install the extension without worrying about the page still loading.
If the version on this page doesn't match the version you're expecting, hold Control and press the Reload button (or press Control-F5) to reload the page.