Firefox keeps on ruling. Even if it wasn't so great in itself, the sheer number of handy add-ons means it's indispensable for serious web-work. The latest and greatest to enter the Poorhouse most-useful collection is AnEC - "Add and Edit Cookies".

This extension allows you to easily view, add, delete and best of all edit the value of any cookie on your system. This makes testing and fixing any site you're developing that involves cookies a ridiculous amount quicker and easier than continual logging-in-editing-something-logging-out-repeat-to-fade type nonsense.