Been trying to fix this for two weeks..
Have a SSL issues from network solutions..chrome was fine, IE was fine and safari was fine..can make transactions.
However FF and Opera still showed untrusted information and proceed at your own risk splash page messages.
I used consoles, dev tools, the test ssl plugin and couldnt find any artifacts that were coming into the cart/checkout pages as http.
By accident found out that my favicon was http! Chrome/IE/Safari don't bother to look at the favicon, even if http as a possible security risk while opera and FF do, and all the tools ive used out there don't list the favicon in whatever console one may use to inspect http vs https..
just thought id share