Exchange Calendar in Thunderbird
When I was searching for a long time to find a good solution to integrate my Microsoft Exchange calendar to Thunderbird, I found this solution. Although Microsoft hasn’t implemented a solution themself, these guys did this job and provide a great solution. It allows you to read and write(!) appointments into your calendar.
If you need to integrate e.g. an Office 365 calendar, you can find some tips for the setup from the original author here.