It is possibly that you need to recover the certificate that is send by your security software.

You can do that by copying the cert8.db from the "Old Firefox Data" folder on the desktop to the current Firefox profile folder.

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

There is security software like Avast and Kaspersky and BitDefender and ESET that intercepts secure connections and sends their own certificate or that incorporates special web shielding features that can block content.

When you use the refresh feature in Firefox then a new profile is created and some personal data (bookmarks and history, form data, tabs) is automatically imported. The current profile folder will be moved to an "Old Firefox Data" folder on the desktop. Installed extensions and other customizations (toolbars, prefs) that you have made are lost and need to be redone.

It is possible to recover data from the "Old Firefox Data" folder on the desktop, but be cautious not to copy corrupted files to avoid carrying over problems.

I copied the Old Firefox data to a brand new profile and that worked a treat.

From the browser's behaviour, I suspected the certificate and good to also know how to just restore that.

Thanks for your help.

Hi Corwin99, do you know if the issue was Firefox not having the "root" certificate for your security software? Since this identical issue of a "man in the middle" can occur with malware, it would be ideal if you could confirm that.

Typically, you just need to call up a certificate on a site and check the issuer name/organization to see if it is something you expect (e.g., Kaspersky, ESET, Avast) or something you don't expect.

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Certificate"
• (menu bar) Tools > Page Info > Security > "View Certificate"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Certificate" button

In the dialog that opens, the "Issued by" section is the key one to check.