In short, a number of poorly-behaved devices reuse the same self-signed SSL certificate, primarily consumer routers and enterprise out-of-band management apps like iDRAC and iLO (and apparently a number of firewalls, judging by the comments on that tracker). Once you've hit a site that presents a cert, Firefox will flat out disallow you from visiting a different site that presents the same (non-wildcard) cert, with no opportunity for override.

The only way to work around this is to nuke your cert DB outright and restart the browser, or to run a command with the cert's fingerprint as an argument to remove it, and restart the browser. The simple option would be a button to just kill the offending cert, which I personally suggested last year (third comment from the bottom) and didn't even get a reply to.

Meanwhile, Chrome, IE, and Edge don't have this problem.

This has been going on for 12 fucking years. Apparently WebVR and shopping plugins are more important than basic usability.

Yeah, I got that concert email too. What the actual fuck is this company even doing anymore?