Although it's possible to create separate login profiles on Windows, it's often inconvenient when sharing a computer with other household members to start a whole new login session when someone just wants to quickly access their webmail or homepage. A more convenient solution is to create separate user profiles in Firefox to manage different homepages, bookmarks, etc.
Firefox's built in profile manager makes creating new user profiles a trivial task. In this example, I'm going to create two new profiles; His and Hers. I'm doing this in Windows XP. The Firefox commands should be the same in other versions of Windows, but the procedure to create the desktop launchers may vary slightly.
Go to Start->Run and enter
and click OK.
In the Profile Manager click on Create Profile. In the new window, click Next. Type a name for the new profile, (I used Mr for mine), and click Finish. Repeat the process to create another new profile for Mrs.
Now we're going to create desktop launchers for each of the profiles. Right click on the desktop and select New->Shortcut. Type "C:\Program Files\Mozilla Firefox\firefox.exe" -P Mr --no-remote for the Location and click Next. The -no-remote option makes it so multiple profiles can be open at the same time.
Next, give a name for the shortcut. I'm using Firefox-Mr. Click Finish.
Repeat the process to create a launcher called Firefox-Mrs.
You now have launchers to use the two different Firefox profiles. You can now maintain separate bookmarks, homepages, extensions, themes, etc.
If you are using Linux, you can Create Multiple Firefox Profiles in Xubuntu.