Rated 2 out of 5 stars

You should have the following clearly visible in the initial description for this plugin - "This version of the Provider for Google Calendar REQUIRES Sunbird/Lightning 0.8." -
Otherwise, people like me who install it will be left wondering why it doesn't work, and the uninformative message displayed is "Requires additional items'

This review is for a previous version of the add-on (0.4). 

I guess its different when on the Thunderbird page, but if you are on the Sunbird page, then it says "Works with 0.8 - 0.8". I'll see what I can do on the Thunderbird page.

Is the display of version numbers your only gripe? Personally I don't think that justifies a 2 star rating.