Step 1
Open Thunderbird and verify you have the latest version installed. Click “Help,” then click “About Thunderbird” to see if you need to update. As of September 2012, the latest version is 15.0.1 and is required to complete the steps below.
Step 2
Click “Tools,” then click “Add-ons.”
Related Reading: How to Export Thunderbird Email to MS Outlook
Step 3
Search for “Lightning” using the search box. As of September 2012, the latest version of Lightning is 1.7 and should appear near the top of the search results.
Step 4
Click the “Install” button associated with “Lightning 1.7.”
Step 5
Click the “Restart now” blue text above the Lightning add-on after installation.
Step 6
Notice that you are now prompted with “Accept,” “Decline,” and “Tentative” reply options for Outlook meeting invitations.
Step 7
Click “Accept” to accept the Outlook invitation. You can now accept invitations from Outlook on Thunderbird using the Lightning calendar add-on, and also have Thunderbird send email notices that you have accepted.