Officially, Google dropped support for the Notebook extension with version 3.0 of Firefox, but you were able to get it working in Firefox 3.5 with this hack.
However, I've been using the Firefox 3.6 Beta for a while now, and the performance is degraded. The extension seems to still allow you to bookmark a page without going to http://notebook.google.com, but the popup doesn't register any mouseclicks.
I'm currently in the process of creating a Firefox dev environment, just to keep this extension working -- that's one way to cut my teeth on Firefox extension development, I guess. :P