Css Not Rendering On Firefox For A Split Second

From what I can tell, when browsing my site using Firefox it seems there is no CSS for a split second on my page (all pages). The page has no style when going from page to page but it is just for a split second and then it load properly. Seems almost like a subliminal message. I tested on a second computer and it does the same thing. All other browsers don't show this delay; just FF. Anyone else seeing this or know what could be causing it?

https://fastdecals.com/shop/