Bugmenot has a bookmarklet, but it opens a new window, and is therefore slow, and you have to copy and paste login information yourself.
Ubiquity is a command line interface to various things for Firefox.
I've created a Bugmenot command for Ubiquity.
bugmenotwhile viewing the desired login page, and the top-ranked Bugmenot entry will be populated and submitted.
If that login has expired, you can specify an entry number as a parameter, from the list of entries in the Ubiquity preview pane.
Get it at http://gist.github.com/102363.
Some code was borrowed from Onur Yalazı and Brandon Goldsworthy.
7/9/09 update: Updated to support Ubiquity's Parser 2. The old code is still available here.