You likely are not using HTTPS, or it is not properly configured.

www.ssllabs.com can help you determine what it wrong, and how to fix it.

observatory.mozilla.org can also help

A padlock with a strike through means that mixed active content is blocked.

You can check the Web Console for more detail about what content this is about.

• https://developer.mozilla.org/Security/MixedContent

Yes it's a http website. Does that mean it can't be fixed

No. It means that it the signed certificate is not set up properly so that it shows the HTTPS address and Padlock. If have a CPanel from your host there are things in there that can help you as well as the provided URL's above.

Does this happen in other browsers ? Also have you deleted your cache and cookies in Firefox since you installed the certificate ?

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?