HTTP site | HTTPS site
This is a simple improvement to the location bar to help users correctly identify website domains.
During normal browsing, most of the URL is faded out except for the most significant part of the domain name. When focus is on the URL field, the whole URL is displayed normally.
No custom icons are shown to the left of the URL. Only the padlock icon can appear there, when SSL is active.
Try clicking inside or outside of the location bar to set the focus.
See the discussion on Bug #366797 or in mozilla.dev.apps.firefox.