The answer is that this was caused by a change to Firefox. "December 2018: Firefox's built-in feed reader and Live Bookmarks have been removed, starting with Firefox version 64."
The solution: There is a Firefox add-on called Feedbro that is easily added and seems to work just fine...
I got home last night to find a voice message on my home machine from an actual human at TicketBastard telling me that the concert was cancelled. It was nice of them to do that, but really an email would have been sufficient.
Perfect! I took the liberty of combining those into a PDF. (I had to reduce the file size to fit within the upload size restriction, but it's still legible.)
UPDATE: See below for better scans below from Ryan and a link to download them as a PDF.
Just got this email from Ticketbastard:
Hi live event fan,
Your event is still on, but it's at a new time.
Thursday, December 7th at 7:30PM NEW TIME: 8:30PM
Your tickets are still good – see you there!