This page explains what you should be aware of as a web developer.
Your website may break
If your website delivers HTTPS pages, all content (image, script, video, stylesheet, iframe, font...) delievered via HTTP on this pages will be blocked by default. Consequently, your website may appear broken to users (if images don't load, etc.).
Note that since mixed content blocking already happens in Chrome and Internet Explorer, it is very likely that if your website works in these browsers, it will work equally well in Firefox with mixed content blocking.