RSS Multi Importer lightbox not working in Firefox

I’m having a problem with the pop up lightbox working. It works in Chrome but not Firefox. I’m on a Mac using Firefox 27.

Here’s my site: Starhorsepax Designs Blog

And here are a couple of the feeds. One of them worked but took a long time, the other never popped up at all.

https://www.thehorseinart.nl/?feed=rss2

https://rtfitch.wordpress.com/feed/

Is this some sort of conflict with Firefox? If it was another plug in I’d rather expect it to have a problem in both browsers.