Great advice give right below, Spa, but I would make sure the date on your computer is set properly first. I just ordered a few shirts from MGA a few days ago with no certificate warnings.
An expired certificate doesn't necessarily mean "unsafe." I set certificates at work and it does often come down to the user's computer date (month and year).
Browser and OS can also give false positives on the popups for certificates. I just checked again and Firefox / Mac (lion) has no certificate trouble there.
