Thunderbird calendar events don't have clickable URLs turned on

Solution 1:

A bypass solution is to add the URL in the "location" field of the event. In that case, when you get the event notification (e.g. 1 minute before the meeting starts) the URL in the location field appears clickable.

This doesn't fully reply to your request, since many events come as invitations from other people, and in that case the URL is part of the description and is not clickable. I think the only solution to that is to highlight the importance of the older bug in in bugzilla and this newer one and hope it gets implemented soo.

Solution 2:

You can put a clickable link in the Attachments tab for the event.