I recently found a nice calendar add-on for Thunderbird called "Google Calendar Tab". It adds Google Calendar to Thunderbird and looks & works just like it does when you use a browser for the calendar. I've tried it on both Linux & Windows versions of Thunderbird.
I've been using the Calendar tab in Thunderbird for a number of months and I really like it. It reminds me of the Calendar function in MS Outlook - which I use quite a bit at work - and, IMO, it's much better than the last Thunderbird calendar add-on (Sunbird?) I tried.
-- Edit --
I just checked and what I have isn't the Google calendar plug-in, it's the Lightning 1.0 Calendar extension for Thunderbird. The assessment remains the same.