Improve Firefox's Responsiveness While a Page is Loading

If Firefox is too unresponsive for your tastes when it's loading a new web page, enter about:config into your address bar and then add the content.switch.threshold setting (which isn't there by default). Right-click the page and select New -> Integer, name it content.switch.threshold, and give it a value of 1000000. The catch is that Firefox will take slightly longer to load pages, but while it's loading you'll be able to scroll the already-loaded content more easily.