Having spent the better part of today debugging a problem with cookies on a couple different servers, I stumbled on the fact that the “Private Browsing” feature of Firefox was exceptionally handy for my situation. Private browsing is usually touted as a feature for “porn browsing”, or other activities in which you’re trying to not leave a trail. The same cleanroom features make it ideal for testing and developing web applications.
In a typical case where you’d need to clear browser history, clear cookies, and so on, I would usually keep one browser set up with developer tools (bugs, twiki, etc.), and launch another browser to be the guinea pig. With private browsing, I can now use my preferred browser (Firefox) for both cases.