I don't have a solution to this, but just FYI, Firefox 76 was released two days ago and removes the certificate install prompt you depend on in openCertificateInFirefox. The FF bug is here: https://bugzilla.mozilla.org/show_bug.cgi?id=1024871.

Responses with Content-type: application/x-x509-ca-cert now just prompt for download, and don't offer to install the cert at all.

I'm trying to solve this independently for a separate project - if you come up with a nice alternative, I'd love to hear about it!