Hi Tad
It's IE8 bug.
Even more problems with the IE 8 mixed content warning
http://blog.httpwatch.com/2009/09/17/even-more-problems-with-the-ie-8-mixed-content-warning/#comment-32970
Try changing embed code explicit HTTPS protocol.
Before
<script src="//app-xxxx.marketo.com/js/forms2/js/forms2.js"></script>
<form id="mktoForm_9999"></form>
<script>MktoForms2.loadForm("//app-xxxx.marketo.com", "999-xxx-999", 9999);</script>
After
<script src="https://app-xxxx.marketo.com/js/forms2/js/forms2.js"></script>
<form id="mktoForm_9999"></form>
<script>MktoForms2.loadForm("https://app-xxxx.marketo.com", "999-xxx-999", 9999);</script>
But this is my little idea. I can not know that your problem will be fixed.
Takehiro