Firefox is the most developer friendly browser.
One feature I use all the time, but is not very well known is the Firefox profiles feature. Essentially, I have different shortcut icons set up on my taskbar to start different Firefox browser windows that have completely different cookie, add-on, and configuration setups. For example, I can have one Firefox window for general browsing and my personal account. In another browser, I can have a test account with test cookies and settings. If I didn’t have this, I would have to resort to using a virtual machine or some other system that would add a tremendous amount of overhead.