When I display a Post in Firefox,
wp_count_comments( $post->ID ) works correctly, but in IE9, it functions as if I had coded
wp_count_comments(), i.e. - counts all the posts on the site. In my case (in IE9), for each post the Loop is looking at, I get "Most Recent Comment (of 4) 0.9 hr. ago" from the code I've written. In FireFox, only Posts with comments show as having comments, with the correct count: "Most Recent Comment (of 2) 0.9 hr. ago".
Here is the test post you can look at in different browsers to see the difference: http://rwcrn.com/jontest
Here is the complete set of code:
Line 35 is where the function is called.
I've tried to recreate this on a test site, but never succeeded, even by exporting and importing all the posts from the real site.