I am aware that some links and features of Google Calendar do not work properly with this addon.
For example, the "sign out" link does not work.
You can sign out by opening the error console (in the menu "Tools" > "Error Console"). Paste the following code into the input field and click "Evaluate":
Components.classes['@mozilla.org/appshell/window-mediator;1'].getService(Components.interfaces.nsIWindowMediator).getMostRecentWindow("mail:3pane").document.getElementById("tabmail").openTab("contentTab", {contentPage: "http://www.google.com/calendar/logout", type: "googlecal"});