Although it's possible to create separate login profiles on both Windows and Linux, 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 Xubuntu 8.04. The Firefox commands should be the same in other versions of Linux or even Windows, but the procedure to create the desktop launchers may vary slightly depending on your desktop environment.
The first step is to open a terminal and type:
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 Create Launcher. Type Firefox-Mr for the name. For the command, type firefox -P Mr -no-remote. The -no-remote option makes it so both profiles can be open at the same time. For the icon, select /usr/share/pixmaps/firefox.png. Now click on Create and the new icon will appear on the desktop. 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.
I've written a Windows specific version at: How To Create Multiple Firefox Profiles in Windows