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: by default, Firefox doesn't try to render a web page for 250 milliseconds while it's waiting for data.
If you use about:config to add the Integer preference nglayout.initialpaint.delay, and set the value to 0, Firefox immediately starts to display the page, even without complete data (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).
