Firefox SSL problems: sec_error_unknown_issuer

One of my clients was reporting that a few of their website users were seeing a warning page when viewing an SSL secured page with this error message:

sec_error_unknown_issuer

It appeared as though it was only a tiny number of people reporting the problem, and all of the users were using Firefox (but not everyone using Firefox was getting the error).

I tried a number of things that didn’t work, eventually re-installing the SSL certificate and that seemed to fix up the problem immediately. If you are using cPanel/WHM, re-installing the certificate is as simple as logging in to WHM, going to the “Install a SSL certificate and Setup the Domain” page, click “Browse” half way down the page, select the certificate that you want (it should pre-load all of the boxes), and click Install at the bottom.