It’s not required to know where your profile folder is located in order to create a new one. However, it’s good to know it anyway, for example, if you want to make a backup of your settings.
On Windows XP/2000, the path is usually %AppData%\Mozilla\Firefox\Profiles\xxxxxxxx.default\, where xxxxxxxx is a random string of 3 characters. Just browse to C:\Documents and Settings\[User Name]\Application Data\Mozilla\Firefox\Profiles\ and the rest should be obvious.
On Windows 95/98/Me, the path is usually C:\WINDOWS\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default
On Linux, the path is usually ~/.mozilla/firefox/xxxxxxxx.default/
On Mac OS X, the path is usually ~/Library/Application Support/Firefox/Profiles/xxxxxxxx.default/
%AppData% is a shorthand for the Application Data path on Windows 2000/XP. To use it, click Start > Run…, enter %AppData% and press Enter. You will be taken to the “real” folder, which is normally C:\Documents and Settings\[User Name]\Application Data.