Firefox page load event very long

I'm going to try to explain an issue that scratch my head...

When I open this website for the first time after opening Firefox (v43.0.3), the UI freeze during about 20 seconds and I don't undersand why. For example you can see that the background image appear after almost 20 seconds...

So, I used firebug net panel to try to understand where the problem comes from and I notice that the page load event is very long but I can't see any tasks executing during this event.

here is a screenshot of firebug Net Panel

When I open the website with Chrome it works fine, does someone has ever encounter that kind of problem ?

I hope that makes sense, thank you in advance.