alert() everywhere has gotten you tired of hitting enter a hundred times on every page refresh.
Enter some really nice web application development tools. Most of these use Firefox, and modify the user-centric application into a developer's tool.
The Web Developer Firefox Extension is probably the most well known/common. It adds a toolbar that allows you to quickly and easily inspect a webpage's layout, disable cookies, view source, modify the css on the fly, or do browser teting. It's indispensable for developing your own site's layout as well as understanding how other site layouts are handled.
The last tool is not Firefox specific, but is a special build of Apple's Safari and KHTML engine which provides a very nice DOM inspector: Safari/Webkit Web Inspector.