How to open multiple firefox windows with distinct sessions? [duplicate]

Possible Duplicate:
Firefox: Using multiple instances with different profiles?

I'd like to open two or more firefox windows and have distinct sessions in each.

For instance:

I open window 1, and go to gmail.com and sign in.

I open window 2, and go to gmail.com - I'm not currently signed in with this window. So I can sign into a different account.

How can I do this?

You should look at the thread mentioned by hyperslug, but to condense it (and add something):

You need to use multiple profiles, together with either:

CookieSwap

CookieSwap enables you to easily swap all your cookies so that you can be logged in to multiple web e-mail accounts (like Gmail and Yahoo! mail) as different users at the same time and quickly switch between them.

MultiFirefox

MultiFirefox is a small launcher utility that allows you to run multiple versions of Firefox side-by-side. It helps you set up multiple profiles (one or more for each version of Firefox), remembers your last launch preferences for easy launching, has auto-updating built in for when we make improvements, and is 100% open source, MIT licensed.