I solved a Problem with Google Calendar and Firefox today, just in case you experience the same. Somewhere last week the web-app of Google Calendar began to display the title bar but not the calendar itself. When opening it with another browser it worked fine.

The solution: I am using Firefox with the (great) add-on Adblock Plus. It turned out that the Filter RuAdList (which I had enabled by default) blocks essential parts of Google's web services. After disabling RuAdList in the Adblock-Plus preferences the Google Calendar worked fine again.

Seems like I got to find another Filter for the Russian web. It should be possible to download the Filter, remove the Google related stuff and use a local copy.