Firefox / Google Chrome plugin to enable multi-session browsing

The traditional way to browse a site with multiple sessions is by using multiple browsers at the same time. Are there any Google Chrome / Firefox plugins that can do the same using a single browser?

I have created a script for Google Chrome that allows multi-session browsing.

Download the installer, or just run the script directly.

It creates a shortcut that launches a completely separate instance of Chrome which has its own cookies. (In fact, it has its own history as well.) This allows you to test your pages with multiple user accounts at the same time, or use multiple Gmail/Facebook accounts, etc.

For Firefox, there is also the Multifox plugin which allows per tab identity (whereas CookieSwap does not yet). Quite handy!

The difficulty in doing what you're asking revolves around the way the session is maintained between client and server. In most web sites, sessions are tracked by a cookie set in the browser, which gets stored in a way similar to your web cache, which is then shared across instances of that browser.

I could conceive of the possibility of creating an addon that would allow you to force which cookie a given window/tab uses, but I'm not aware of such an addon today. The closest thing I can find in a few minutes' searching is CookieSwap for Firefox, but it hasn't been updated in years.

The CookieSwap^* firefox extension is pretty useful.

If you have multiple logins for web based e-mail accounts (like Gmail and Yahoo! mail), then CookieSwap enables you to easily switch between those different accounts by swapping the 'cookies' that the sites use to know you.

^* Not available for Firefox 4 yet, but top of the page it says it's coming soon.