From GreaseSpot

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

Windows 7 

C:\Users\UserName\AppData\Local\Mozilla\Firefox\Profiles\Random Value.default
or
C:\Users\UserName\AppData\Roaming\Mozilla\Firefox\Profiles\Random Value.default

Windows 

C:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles\random value.profile name
Note: you may need to enable displaying hidden files to see Application Data, or type it into the address bar.

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