Finally! I have been waiting for a Google Calendar extension for Firefox and it is finally here. This extension will notify you of upcoming events and will even let you add an event into your calendar. It currently only supports 1 calendar but they do have plans to support multiple calendars in a future release.
Overall, I would say that this extension was very well written and the future that an extension like this has is amazing. This extension, if continually updated with features, could become one of the most popular Firefox extensions because of the popularity the Google Calendar now has.
On a side note: those people that use Google Bookmarks make sure you check out this extension that was released a short time ago.