Consider a setup where all HTTP/HTTPS traffic goes through a proxy. The certificate of this proxy must be trusted by all browsers (in this case, Firefox). This certificate replaces the original certificate for all HTTPS connections to hosts which are not in a whitelist maintained by the sysadmin. As such, all HTTPS traffic to non-whitelisted hosts can be inspected at the proxy.
To check whether a connection has been whitelisted, I can simply right-click and go to View Page Info → Security to see the "Verified by" field. This is a bit of a cludge, and I don't want to have to go through these steps manually for every HTTPS connection. So how can I display the certificate issuer for the current connection at all times in Firefox?