The version 2.0 of Google Calendar Notifier Firefox extension is reportedly 'currently in closed testing'. Version 2.0.4 Beta was available for download since 9th May. The new version will support multiple calendars.
More information at author's forum: http://extensions.npike.net/forums/

The recent version available via Firefox Add-ons site is v1.3.
This .xpi extension was announced here in April
http://blogoscoped.com/archive/2006-04-27.html#n24