Just remember to right-click on the green 'Install Now' button to download the add-on otherwise you will install it into firefox. I installed the add-on and restarted Thunderbird and now the raw vCalendar email was transformed into a nicely formatted meeting invite - you even get the option to accept or decline the meeting. More information can be found at Lightning's home page. This adds a calendar into Thunderbird and also adds support for understanding vCalendar events. His suggestion was to install a Thunderbird add-on called Lightning. After a quick google search I found a post on Daniel Tome's blog regarding Viewing Calendar Events in Mozilla Thunderbird. You can still use Thunderbird even though the calendar functionality does not come built-in as it does with Outlook.

There is no need to run MS Outlook just because you need to use an electronic calendar with your email program. I received an Outlook meeting invite in my email recently but because I use Thunderbird as my email reader I received the raw vCalendar data rather than a nicely formatted email with details of the meeting. How to Use Mozilla Thunderbird as Your Calendar Application at SU.

Thunderbird outlook calendar