Problem displaying favorite button in Firefox

Hi there,
I’m having a problem with this plugin in Firefox. I’m generally testing my site using Chrome Dev tools and everything has worked fine. However, I opened the site up in Firefox (latest version) and the Favorite buttons aren’t showing up. Inspection in Firebug looks like they aren’t getting added to the pages.

I did a basic tweak of

function get_favorites_button($post_id = null, $site_id = null, $group_id = null)
{
global $blog_id;
if ( !$post_id ) $post_id = get_the_id();
if ( !$group_id ) $group_id = 1;
$site_id = ( is_multisite() && is_null($site_id) ) ? $blog_id : $site_id;
if ( !is_multisite() ) $site_id = 1;
$button = new FavoriteButton($post_id, $site_id);
return $button->display();
}

replacing it with a simple echo of “test” and it showed up normally. Can someone advise me what might be causing the buttons to fail in Firefox, or give me some steps to debug further?

Thanks!