Certificate comes from the site or connections you use to connect to another site to verify the site is legit and not a malware site. As to your previous employer no one here can verify or know what they did. Why would you let them use your personal laptop for?

Then you need to check the Firefox help link.

"How do I get to the certificate manager for my firefox browser so I can manage the certificates of sites I frequently visit?" https://support.mozilla.org/en-US/questions/999481#answer-571230

Otherwise you need take it to a computer shop to diagnosis the problem. You screenshot shows a "delete" option that should remove any certificate that no longer needed. So unless this is company computer at that point that's their property and they can see and view what your doing with their computer.

Hi David, the certificates under the "Your Certificates" list are typically only relevant for a specific site where your device needs to prove it is authorized to connect to that site. If the computer was set up by an IT department, that could explain why the certificate is there. You could back it up and delete it if you don't think you need it for any sites you use.

As for Facebook, have you already changed your password? In your account settings, you can check whether any apps or websites are connected to your account and remove/deauthorize them:

https://www.facebook.com/settings/?tab=applications