From MozillaZine Knowledge Base
Rather than wait until a page has completely downloaded to display it to the user, Mozilla applications will periodically render what has been received to that point. Because reflowing the page every time additional data is received greatly slows down total page load time, a timer was added so that the page would not reflow too often. This preference specfies whether that timer is active.
Possible values and their effects
Don't reflow pages at an interval any higher than that specified by content.notify.interval. (Default)
Reflow pages whenever new data is received.
First checked in
Has an effect in