Found a fail condition on Firefox
Not logged in - add items to cart - click login in the cart page header - logs in, but the checkout button goes "the requested URL was not found on this server. The link on the referring page seems to be wrong or outdated."
The checkout button on the cart page, and in the header quick cart, were redirecting to the checkout page up to that point. Unlike in IE, I am able to use my logout button to clear session, which gets the site back to normal - everything works fine until I run that one scenario: add items (not logged in) - login from header on cart page - then use checkout button.
My access to checkout.php is "logged into users"
Page redirect from cart is checkout.php
If access denied to checkout.php "login.php?checkout=1"