Or use a free online crawler like SSL-check or Missing Padlock, a desktop crawler like HTTPSChecker, or a CLI tool like mcdetect to check your website recursively and find links to insecure content.
In any case, the best way to know if something is broken in Firefox is to download the latest Firefox Edition, open different pages on your website with the web console open (enable the 'Security' messages) and see if anything related to mixed content is reported. Note that since mixed content blocking already happens in Chrome and Internet Explorer, it is very likely that if your website works in both of these browsers, it will work equally well in Firefox with mixed content blocking. Passive mixed content is displayed by default, but users can set a preference to block this type of content, as well. Consequently, your website may appear broken to users (if iframes or plugins don't load, etc.).
Google Chrome, Microsoft Edge, Mozilla Firefox, and Apple Safari are all browsers with. If your website delivers HTTPS pages, all active mixed content delivered via HTTP on this pages will be blocked by default. Some even tweak the warning to your connection is not secure. Mixed Content refers to the various components of a webpage (iframes, active scripts, hyper links, images) being served over unencrypted connections (HTTP).
