I just noticed this one on one client's osCommerce installation yesterday. I went in and simply modded application_top so that it forced it to not use the SSL hostname (which is set to '' because there is no SSL hostname due to the fact all transactions are done off-site via PayPal).
I received an E-Mail from ANOTHER client using osCommerce today (again, no SSL cert, no SSL url as their checkout is done via PayPal).
It seems that it's the base= line in the HTML header output.
It's outputting as base="" (as there is no SSL url defined in the config file), so it's assuming it's in SSL mode when it's not).
IE seems to take base="" as top priority, and thus attempts to load
http:///image.jpg
FireFox ignores base="" and loads the images current to the relative path of the page (
http://www.domain.com/image.jpg).
As 90% of the internet still uses Internet Explorer, this is a pretty big deal.
Oh yeah, all their config files are defined as follows..
define('ENABLE_SSL', false); // secure webserver for checkout procedure?
define('HTTPS_SERVER', '')
And this is with MS1 and MS2 installations.