I have to use a site called "Blackboard" for my college courses. Whenever I go to it, it gives me a security certificate pop up window. The site is secure but I keep having to add as an exception multiple times daily. The "permanently add to exceptions" box is greyed out so I can't check it. On the PC's at school, they are using the same version of Firefox that I am but the box is not greyed out, so it will let you check it. I am using the latest release.
Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.
(If you're not using it, switch to the Default theme.)
Once you get the pop-up, just select "'Start in Safe Mode"
If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.
To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.
When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.
Make sure that you do not run Firefox in (permanent) Private Browsing mode.
You can also check if that page is opened in an (i)frame.
In that case you should have a "This Frame" item in the right-click context menu and you can try to open that frame in a new tab.
Check out why the site is untrusted (see the Technical details) and if this is caused by a missing intermediate certificate then see if you can install this intermediate certificate from another source.
You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.
Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".
You can see more Details like intermediate certificates that are used in the Details pane.
Same problem for me. I created a self-signed certificate on my personal home server (https://pieroland.linuxd.org/). When I try to add an exception to Firefox (I understand the Risks -> Add Exception...) the browser says (in the pop-up used to add the exception):
Valid Certificate This site provides valid, verified identification. There is no need add an exception.
The "Confirm Security Exception" is greyed-out and I can't access my pages
I can reproduce the problem when running in safe mode, and I'm not using private browsing. It works fine in Chrome :-(
Yes I removed them, but didn't work.
By the way, it's Firefox 22.0 on OSX 10.6.8.