I discovered a Javascript error that was preventing the popup from appearing in Internet Explorer and was preventing me from changing the text of the close button.

In mod_moopopup.php, line 83, the 'btnTitle' key was present twice. The second instance was overriding the first, meaning that the close button text was always 'close'. The first instance was not wrapping the $buttonTitle variable with escaped quote marks, which as causing the error in IE.

I removed the second 'btnTitle' key and replaced the first one as below. Now everything is working correctly:

btnTitle:''.$buttonTitle.''