Google Calendar / Google Apps integration with Thunderbird, seems to be a simple task to accomplish don't you think?
This is the second or third time this has happened to me therefore I thought I'd share this with you guys!
Well, I had an experience with one of my colleges today having to install Thunderbird + Lightning + Google Provider on Ubutnu 9.04 and I tell you it wasn't that straight forward.
The recipe is supposed to be quite simple if you want an external calendar in your Thunderbird you need lightning, if you want to integrate a Gmail calendar with your Thunderbird you need Google Provider.
In fact, what happens is if you install Google-provider & lightning from apt-get (or Synaptic), you cannot sync your calendar with Thunderbird and you keep getting errors like "Your calendar is in read only mode" etc etc, which sends you to your calendar settings - but the answers are probably not there!.
A second option is going to Thuderbird add-ons downloading both Extensions and installing them which also fails, for the same reason.
This is all caused because you probably installed Thuderbird by apt-get install Thunderbird and I haven't figured out the difference yet, but as a fact apt-get interferes in the way Thunderbird (and firefox too) are installing / enabling the plug-ins and I am pretty sure this is the cause of this problem.
So what worked for me:
Install lightning from Synaptic- at this stage Synaptic automatically adds Google-Provider and Calendar-Timezone package - install them all!, for you cannot install lightning without them, then Mark the Google-Provider extension for "complete removal" (it shouldn't, but make sure this doesn't remove the lightning extension).
Install Google Provider from Thunderbird's add-ons - in Thunderbird go to tools > add-ons > "Get Extensions" which should open a browser look for Google and download the provider_for_google_calendar-0.5.2-sb+tb.xpi file - in the add-ons screen in your Thunderbird install the xpi file
Disable the Timezone Definitions for Mozilla Calendar - this seems to cause an issue with the timezone settings and results in sync failure with your Google calendar.
Once you have completed the step above restart Thunderbird.
Add the Calendar of type Google Calendar, restart Thunderbird.
Once you login Thunderbird should be displaying your calendar correctly.