From MozillaZine Knowledge Base
Mozilla applications render web pages incrementally - they display what's been received of a page before the entire page has been downloaded. Since the start of a web page normally doesn't have much useful information to display, Mozilla applications will wait a short interval before first rendering a page. This preference controls that interval.
Possible values and their effects
The number of milliseconds to wait before first displaying the page. (Default: 250)
Lower values will make a page initially display more quickly, but will make the page take longer to finish rendering. Higher values will have the opposite effect.
First checked in
Has an effect in