From MozillaZine Knowledge Base
Mozilla applications store a user's personal information in a unique profile. The first time you start any Mozilla application, it will automatically create a default profile; additional profiles can be created using the Profile Manager. The settings which form a profile are stored in files within a special folder on your computer — this is the profile folder. The installation directory also includes a "profile" folder but this folder contains program defaults, not your user profile data.
For information on the profile folder specific to certain applications, including how to find the profile folder, see the following articles:
For other applications, see the information below. Some of these folders may be hidden.
For Windows users
On Windows systems, the default location of the profile folder containing your user data is under the "Application Data" folder in Windows XP and below or under the "AppData\Roaming" folder in Windows Vista and above. In Windows 2000 and above, these folders are hidden. To find your profile folder in Windows 2000/XP/Vista/7/8/10:
A Windows Explorer window will open, showing the contents of the C:\Documents and Settings\<user name>\Application Data folder (Windows 2000 and XP) or the C:\Users\<user name>\AppData\Roaming folder (Windows Vista, 7, 8 and 10). You can then open successive folders until you get to the profile folder.
If you want to browse to the default profile folder locations for Windows 2000 or later you will need to enable viewing of hidden files and folders in your Windows Folder Options. You can also search for specific files in your profile folder using Windows Search. In Windows 2000 you must first enable viewing of hidden files and folders; in Windows XP and Vista, you must enable searching for hidden files and folders in the Search tool itself. If you need more help viewing or searching hidden files and folders in Windows 2000 or later, see this article.
Make sure that you do not have read-only or locked files in your profile, which might be a side effect of backing up the profile to removable media and then restoring your profile from that media. Having such write-protected files in the profile can result in serious problems, as described in bug 257288. Note that, on Windows, you can remove read-only attributes at the directory level for all subordinate folders and files, through the file properties of a folder.