Security Warnings in Firefox related to sec_error_unknown_issuer in Webmin

Recently I was recommended by my good friend Mark to try out Firefox (he works at Mozilla currently, so not at all biased) after installing it i went to each of my bookmarks to check how they performed in Firefox.

When I got to my Webmin control panel for one of my servers, I found I received a security issue. Which was odd, as it displays fine in Chrome and Safari, and I had been happily using it for months.
I then thought it may be related to the fact I was using the nightly version of Firefox, however the standard Firefox version was also throwing up the same error, as you can see in the below screenshot:

After speaking to Mark about this issue, and the fact he was not getting the same problem, we had narrowed it down to the fact that Firefox requires the Intermediate or Chain file[1] also uploaded and added to the VirtualHost where it used.

The reason that Mark was not seeing the same error, was because by visiting another website he had installed the 'Intermediate certificate' [2]

For me, this arrived within the Zip sent to me from Comodo:

I then tried various things to add it to the ViritualHost within Apache, until I realised that Webmin uses its own version of Apache to work, therefore I went to Webmin Configuration>SSL Encryption and added the path for my Chain Certificate to  the 'Additional certificate files' area:

After saving that page, visiting the Webmin control panel went through without a hitch!

Hope this helps someone!

Sources:

  1. Comodo support about installation of certificates
  2. Mozilla support regarding the same issue