If Firefox is too unresponsive for your tastes when it's loading a new web page, the How-To Geek weblog suggests a simple tweak to improve responsiveness.
First, 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. If you like that, you may also want to tweak Firefox's rendering speed or check out some of the best Firefox config tweaks.