Comment 8 for bug 819558

Tested this just now and it works a treat. For the interested, the URL I used was this:

https://red003.mail.apac.microsoftonline.com/EWS/Exchange.asmx

and I provided my account details to Thunderbird (full email address + password), and the calendar loads. Understandably, this hostname will change depending on where your account is being hosted.