Wow, I can't believe that I found it. I am really just a newbie with php code. Not that I'm trying to toot my own horn here, I just can't believe that it only took me 30 minutes to find and fix it. It should have taken me hours and hours plus countless Google searches.
Anyways, for those that are interested, it turns out that the Edit Thumb text was wrapped in a <span> with a class="custom_thumb" which it turns out was also defined in the main wp-admin/admin.php file and it was set to hidden. For whatever reason FireFox ignored that but IE didn't.
The fix was to change the span class to something other than custom_thumb. So open up wp-conten/plugins/nestgen-gallery/admin/manage-images.php and round about line 401 you will find this line;
$actions['custom_thumb'] = '<a class="thickbox" href="' . NGGALLERY_URLPATH . 'admin/edit-thumbnail.php?id=' . $pid . '" title="' . __('Customize thumbnail','nggallery') . '">' . __('Edit thumb', 'nggallery') . '</a>';
Change the 'custom_thumb' to something else (I chose cust_thumb), save and upload and test.
Let me know if there are any problems.