When your SSL certificate fails to encrypt all of the elements on a page (for example: forms, textboxes, images,URL's etc.), customers receive an "parts of this page is not secure such as images"
message in there Firefox browser.
That error will show up if parts of the website are referencing images and resources that are using a standard url hosted elsehwere. So say you have a image that is being pulled from http://somelink.com/imagehere.jpg, you will need to update the link to https://somelink.com/imagehere.jpg to show up as secured.
The best way to address this issue is to force the site to always use SSL. You have your site at "domain.com", and you just loaded your site by going to "https://domain.com/". When this issue happens, it means at least one element on the page is loading with a URL of "http://domain.com/image.jpg". Since this "image.jpg" file is loading with "http://" instead of "https://", it will not have the data transfer encrypted. Since not all of the elements on the page are sent via an encrypted channel, your browser will notify you of this as it may be an issue if you rely on having all the information encrypted.
If you have a Wordpress website, you can simply install a plugin called Really Simple SSL that will automatically detects your settings and configures your website to run over https.