Progressive rendering is the act of displaying each object as it is downloaded. Both IE and FF supports progressive rendering; but, they differ on how they render tables.
Internet Explorer 7
When Internet Explorer renders a , it downloads all the objects within the table before displaying it. This is required so that Internet Explorer can render the table with the correct width for each column.
On the other hand, Firefox renders all objects progressively regardless if it's in a table. That is to say - each object is displayed as soon as it is downloaded.
What You Can Do To Improve Performance
Ensure HTML code is valid.
Minimize number of nested tables.
Never wrap entire body contents within a table.
Move towards a CSS-based layout.
Pay close attention to the size of objects and their locations?
The Key Is To Understand
By knowing each browser's strengths and weaknesses, you can build a better design. A good design will never make the reader wait.