From GreaseSpot

The profile directory refers to your Firefox User Profile. It is located at a different path, depending on your platform:

Windows 

C:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles\random value.profile name

NOTE: If you can't see the 'Application Data' map: Extra -> Mapoptions -> Visibility -> Show hidden files.

OS X 

~/Library/Application Support/Firefox/Profiles/random value.profile name
Note: random value.profile name is sometimes "default.dcc"

Linux 

~/.mozilla/firefox/random value.profile name

Above, random value will usually be something like "928za141", and profile name is usually "default" if you haven't created a new profile with another name.

[edit] See Also

• Firefox Help: How To Manage Profiles