Firefox won't allow connection when site certificate is invalid

[Patrick O'Callaghan]

I need to use Firefox to connect to a specific site which has an
unrecognized SSL certificate (the specific error is "Peer's Certificate
issuer is not recognised"). I get the usual dialogue asking if I accept
the risks, but clicking on the Add Exception button does nothing. This
is FF 37.0.1 on F21.

This specific site only works with FF and IE. It used to work with
previous versions of FF, but I last used it weeks ago so I can't be sure
which exact version introduced the breakage. I've already tried running
in safe mode, to no avail.

Has anyone else seen this?

poc