Tempted by post on official google admins blog http://googlewebmastercentral.blogspot.co.uk/2014/08/https-as-ranking-signal.html it's about higher ranking when having https. So I have recently bought cheap ssl certificate from known issuer COMODO and upgraded my server with it but it caused several problems when I redirected from 80 to 443 some 3rd party apps (like userena for django) created loops but I found solution quickly it was to put USERENA_USE_HTTPS=True in settings.py but it still sends more forgot password mails than it should but it's different story.
Comming back to point if you are experiencing error "sec_error_unknown_issuer" in firefox after implementing COMODO Positive-SSL certificate this solution may help you. Don't know exactly but I read that it has something to do with chain certificates they should be bundled inside you website certificate. And remember you have to do it in right order because first one should be your servers certificate cause it's signed with key) and nginx or whatever you use may have problems after reloading service.
My command for bundling was:
cat exerceo_pl.crt COMODORSADomainValidationSecureServerCA.crt COMODORSAAddTrustCA.crt AddTrustExternalCARoot.crt > exerceo_pl.bundled.crt after that I had just to point my nginx.conf to new bundled certificate. And after all reaload of nginx fixed my unknow issuer firefox problem.