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 controls the maximum number of times the content will do timer-based reflows. After this number has been reached, the page will only reflow once it is finished downloading.
Possible values and their effects
No limit on timer-based reflows (Default)
Don't do any timer-based reflows.
Any positive integer
The maximum number of timer-based reflows to perform.
First checked in
Has an effect in