aakk9999 - 11:23 pm on Jun 25, 2013 (gmt 0)
Are you sure that Firefox shows http in the address bar, or is it that it only does not show the message "Only secure content is being displayed" ?
Do you internally link with the full URI or you use relative or root-relative paths (without the scheme/domain name)?
On both, IE and FF, when you are on your checkout page that uses https and then you hover over your other links that should be http, what URL address does the status bar show: https or http?
If https is shown in the status bar, then clicking on that link will go to https version of the page, unless there is a redirect employed. So to make it go to http, there should also be the redirect the other way around, that is, if the schema is https AND it is NOT the checkout page, redirect to http version of the page.