I ran into this issue when trying to get to one of my companies intranet sites. Here is the solution I used:
about:config
into the firefox address bar and agree to continue. security.ssl.enable_ocsp_stapling
.This will lower your security as you will be able to view sites with invalid certs. Firefox will still prompt you that the cert is invalid and you have the choice to proceed forward, so it was worth the risk for me.