From MozillaZine Knowledge Base
Mozilla applications store your personal settings, added extensions and themes, and user data such as bookmarks, passwords, cookies and mail in a "profile". The Profile Manager allows you create and manage profiles. A new profile is useful for troubleshooting since it allows the application to run without extensions, themes, or customized settings. If you have multiple profiles you can use the Profile Manager to switch profiles.
Care must be taken when creating and deleting profiles, to avoid loss of valuable data. This potential data loss may include non-Mozilla files. Be sure to read the warnings given below, here and here, before proceeding!
Accessing the Profile Manager
Caution: Do not use -profile manager (do not add a space). This previously caused a " already running but is not responding" error.  Currently, due to the fix for bug 531532, the command line argument
Close the application and make sure that it is not running in the background.
SeaMonkey or Mozilla Suite: As an alternative to using "Tools -> Switch Profile..." menu option (e.g., if SeaMonkey is not open) you can use the "Profile Manager" shortcut located in the Windows Start -> Programs menu (if available). You can also use the instructions given below for Firefox, substituting
Thunderbird: Use the "Profile Manager" shortcut located in the Windows Start -> Programs menu (if available). You can also use the Firefox instructions given below for entering the full path to the installation directory. For example, enter the following in the Windows Run box (you should substitute the actual path):
Firefox: See Creating a new Firefox profile on Windows for a step by step guide with screenshots, or follow these instructions:
Open the Windows "Start" menu, select "Run" (on Windows Vista, use "Start Search" or enable the Run box, as described here) then type and enter one of the following:
For a zip install, if you have multiple installations (e.g., two different Firefox versions) or if the above instructions do not work, use the full path to the executable (.exe file) surrounded by quotation marks in the "Run" (or Vista "Start Search") box, as in these examples (the second example is for 64-bit Windows):
Note that the above examples are default installation directory locations. You can also find the executable file location by right-clicking the desktop (or Start menu) icon for your Mozilla program, selecting "Properties", going to the "Shortcut" tab, and then copying the "Target" location, as shown in this Firefox screenshot (from the Shortcut to a specific profile article).
Close the application completely and make sure that it is not running in the background. Open the terminal and execute
Alternately, in a terminal, type:
Mac OS X
Close the application completely and make sure that it is not running in the background. Assuming the program is installed in the "Applications" folder, launch Terminal ("Applications -> Utilities -> Terminal") and enter the command starting with / after the prompt in Terminal:
Adding -bin after the application name is necessary on some systems and application versions.   If you have problems starting the Profile Manager with one of the above commands, try again using just the application name, as in this example for Thunderbird:
Creating a new profile
A new profile folder will be created in the default folder location. If you created the new profile because your old one was corrupt, you can import bookmarks and selectively transfer your other data to the new profile.
Note: If you use ZoneAlarm Extreme Security or ZoneAlarm ForceField, the "Browser Security" option to run your browser in virtualization mode, if selected, can prevent new Firefox profiles from being saved. Turn off the virtual browsing feature, if you use one of these ZoneAlarm products. 
Custom profile location
For advanced users only
Mozilla Suite and SeaMonkey 1.x
If you wish to choose your own profile folder location, choose the parent folder where you want the new profile folder created. For example, if you want a new "ProfileName" folder created under D:\Mozilla\Profiles with the profile folder path D:\Mozilla\Profiles\ProfileName\<random>.slt, choose the Profiles folder.
Firefox Thunderbird and SeaMonkey 2
Unlike Mozilla Suite and SeaMonkey 1.x, a new profile folder is not automatically created when you choose the folder location for your new Firefox, Thunderbird, or SeaMonkey 2 profile. Whatever folder you select will become the profile folder. If you want to choose a custom location for your new profile, do not select a folder that contains existing data. You must create a new, empty folder (preferably with the same name as the new profile) and then choose that folder in the Create Profile Wizard. Note: If your operating system does not provide a "New Folder" button within the Create Profile Wizard, you will need to manually create the new folder.
Warning: If the folder you select for the new profile contains non-Mozilla files (such as the "My Documents" folder on Windows), your profile data will be intermingled with the non-Mozilla data. This can result in the loss of all of the data in that folder, including non-Mozilla files, if you later delete the profile. (Bug 304290, Bug 302087).
Deleting a profile
You can use the Profile Manager "Delete Profile" feature to remove a profile from the list of available profiles. You can also delete the associated profile folder and its contents if you select the "Delete Files" option but it is recommended that you choose the "Don't Delete Files" option unless you are sure that the profile folder contains no valuable data.
Deleting a Firefox Thunderbird or SeaMonkey 2 profile
Warning: The folder for the profile you are planning to delete may contain non-Mozilla files, if you created the profile in a custom location (see above). If you use the "Delete Files" option to delete that profile, the entire folder and all of the contents will be deleted, including any non-Mozilla files it may contain. This cannot be undone! For this reason, you should choose the "Don't Delete Files" option when deleting a profile. If you want to delete the profile folder, you can do that manually.
Moving a profile
Renaming a profile
You can use the Profile Manager "Rename Profile" feature to rename an existing profile but this is not recommended. Renaming a profile changes its name in the Profile Manager list of available profiles but does not change the actual profile folder name. If you use multiple profiles, renaming a profile can make it more difficult to tell which profile belongs to which profile folder without taking additional steps, such as checking the profile folder contents or looking inside the profiles.ini file and comparing the Name and Path.
If you do rename a profile, any shortcuts (see below) must be changed accordingly. The profile name is case-sensitive and using spaces in a profile name will require enclosing the profile name in quotes in a shortcut. This can be a problem if a specific profile is called by an extension such as "Launchy" where the options don't allow quotes, so is best to avoid using spaces.
Profile Manager shortcut
These instructions are for Windows. For Mac OS, see shortcut to launch a specific firefox profile on mac
In some Windows installs, a "Profile Manager" shortcut is placed on the Start menu programs list. For easier access to the Profile Manager, you can also create your own shortcut and place it on your desktop or in another convenient location.
For instance, to create a shortcut to the Firefox Profile Manager on Windows XP:
Note: Using -P instead of profilemanager may also work, like so, using the same Firefox example:
Double-clicking the new shortcut should now start the Profile Manager, assuming Firefox is completely closed and not running in the background, as mentioned above.
Shortcut to a specific profile
Follow the above instructions but use -P (not -profilemanager). After the -P add another SPACE and the name of the profile in quotes, where "Profile Name" is the name of the profile you wish to open, as shown in this Firefox example:
Note: Profile names are case sensitive. See the article Shortcut to a specific profile if you need more help.
Profile in use
If the Profile Manager tells you that the profile is already in use, or if you attempt to start your Mozilla application and see a message that the application is already running but is not responding, this means the profile is locked. See Profile in use for details.
Profile missing after using Migration Assistant - Mac OS
If you use Apple's Migration Assistant to transfer all of your files, programs, and settings to a new Mac computer and you see a "Profile Missing" dialog box which says: Your Firefox profile cannot be loaded. It may be missing or inaccessible. when attempting to start Firefox or the Profile Manager, see Profile missing after using Migration Assistant on Mac OS for a possible solution.
Related bug reports