From MozillaZine Knowledge Base

This article explains how you can start with a specific profile when you have multiple profiles for your Mozilla application.

Profiles listed in the Profile Manager

When multiple profiles are listed in the Profile Manager, the last selected profile is automatically used, unless you uncheck the "Don't ask at startup" option. If you don't wish to always see the Profile Manager at startup, you can use the -P "<profile name>" command line argument, either directly or from a desktop shortcut, to start your Mozilla application with a specific profile. See Shortcut to a specific profile for details.

Bypassing the Profile Manager

You can use the -profile "<path>" command line argument to start Firefox, Thunderbird, or SeaMonkey 2 with any specified profile, whether or not the profile is listed in the Profile Manager. This can be useful when recovering a profile that is missing from the Profile Manager, which could happen if the profiles.ini file was deleted or incorrectly edited. You can also try out a profile you've copied to a new location or test a profile backup before adding it to the Profile Manager in this way (see Moving your profile folder for more on that process). See Bypassing the Profile Manager for details.

See also

• Using multiple profiles - Firefox
• Opening a new instance of your Mozilla application with another profile