CookieSwap a lightweight Firefox extension lets you create several distinct cookie profiles in Firefox. Just right-click on the profile button and choose the profile you want to use for each session. Any cookies you download during your browsing session will be saved to your current profile. So for example, if you sign into Gmail and check the box for Firefox to remember your login information, it will be saved to the current profile. You can then switch to a different profile if you want to login as a different user.
CookieSwap works reasonably well, but it's a bit rough around the edges when it comes to tweaking or adding profiles. If you click the "manage profiles" button all you'll get is the directory where your profiles reside. You'll have to navigate to that directory to manually rename your profiles or to add extra profiles if you need more than three.