This is one of those things that are obvious, but nobody thought about, like multitouch scrolling.
As a browser loads pages, the top of the page is in a fixed position, even if it's off the screen. That means that as ads or images load and the page becomes longer, the page on your screen will "jump". This is extremely irritating if you're about to click on a link, and end up clicking on something else.
They should just make browsers fixed about your current screen, or if that is changing as well, about your mouse cursor. Who cares about images that are loading if they're not displayed? This way, you'll never click on the wrong link because your browser loads just as you click.