Tiens peut-etre une idée de pourquoi Fx fait un truc bizzare :

Myth - "Firefox supports Progressive Rendering and Internet Explorer does not" - Example

Reality - "Actually this is not true and you can see from going to many pages that Internet Explorer does support progressive rendering of content as it arrives. This is true however for table rendering. When Internet Explorer encounters a table it measures all the content of the table before rendering so that it knows what the widths of the columns are to render the content correctly. On the other hand Firefox uses a different algorithm in that it renders the table contents progressively before it has all been passed. There are pros and cons to both approaches. In the case of progressive rendering a table it can result in an experience where content is initially displayed and then moved as the browser progresses creating a clunky and poor quality feel. On the other hand if we parse the entire table content first then it can take some time to display in the case of heavily nested tables. I've heard user feedback supporting both arguments and more than a few people have mentioned that they find Firefox's rendering a little off putting in this regard." - Source