Many loungers use the userChrome.css mod to color the background of the address bar when visiting a secure site. I came across a similar mod courtesy of Tech Republic that works instead of, or in addition to, the userChrome.css coding. It's an about:config modification that makes an https site stand out even more. It is like the Extended Validation site identity button but colored blue instead of green. I've set Firefox to color the url bar yellow and show the extended identity button when visiting a secure https site.

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