Jon... I think I have your answer!
I am not directly sure why it would work on some sites and not others but we had another client running into this and I was able to research it more. In the mean time I fixed the issue with Google Chrome and Drag/Drop and also Safari with Drag/Drop for the upcoming 3.0 version of Dynamic Forms!
So... Basically the deal is that some browsers can let you define the inner HTML with just something like TotalCombo.innerHTML = 'test'; etc... But for FireFox you need to have it wrapped around the document.getElementById (careful, its case sensative)!
So... Instead of...
TotalCombo.innerHTML = 'Stuff';
Try
document.getElementById('TotalCombo').innerHTML = 'Stuff';
Hope this fixed you up!
-Chad