Was looking into this for a client, and I’ve come to the following conclusion based on various reading across the ‘tubes:

How cookies are handled between browser instances varies between web browsers. Why do we care? Well various web applications are going to get wonky if you try opening multiple instances of them when those instances share cookies. And by “wonky” I mean it’s just not going to work. So isolating browser instances allows us to have multiple sessions of that web application open simultaneously.

Internet Explorer

IE7 does *not* share cookies if you start another instance of it (e.g. double-clicking on the icon when an instance is already open) but will share them across tabs or if you use “New Window” to open new window.

IE8 *does* share cookies between instances by default, but it can be made to not do this by either:

– Going to File–>New Session

– Starting IE8 with “iexplore -nomerge” (custom shortcut)

Mozilla Firefox

It would appear Firefox shares cookies between tabs and windows if those windows are created under the same Firefox profile. If you’re like me (and using Firefox), you probably only have one Firefox profile setup for yourself. You can force Firefox to use a different profile by creating a custom shortcut that looks like this:

firefox.exe -no-remote -p “myProfile2”

where myProfile2 is the name of the profile you want Firefox to use. If the profile does not exist, Mozilla will bring up the profile management tool which will let you create it. From then on you can then open two instances of Firefox, running under two different profiles, which will *not* share cookies and, thus, will allow you to run two simultaneous sessions of your favorite web application (I know what mine is).

Chrome

Allegedly, Chrome shares cookies between instances unless you use its Incognito feature by clicking on the wrench and going to “New Incognito Window” (Ctl-Shift-N).