Firefox comes in two or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And one folder in the profile of each user on the computer.

If you remove the Firefox folder, the user folders would not be affected.

Only the Firefox programs are stored in the programs folder. User information is stored in the user's profile folder.

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Maybe you are asking about prefs.js

Also note you may well need to see hidden files and full file extensions, something Windows may not do by default. The file prefs.js is in the Firefox profile path not the programs path. There is an easy way of opening that from the troubleshooting page whilst Firefox is open as explained in Fred's post. See also

• http://kb.mozillazine.org/Prefs.js_file
  • http://kb.mozillazine.org/Finding_the_profile_folder_on_Windows#Viewing_hidden_files_and_folders
• Use the Troubleshooting Information page to help fix Firefox issues
• Profiles - Where Firefox stores your bookmarks, passwords and other user data
• Reset Firefox preferences to troubleshoot and fix problems
• Refresh Firefox - reset add-ons and settings

Your fastest and simplest solution will probably to do a Firefox Refresh BUT note that will remove Firefox add-ons so you may wish to bookmark the home sites of any addons you have. Also you seem to be way out of date and using Firefox 40 you should by now be on Fx48 with Fx49 due out next week

• See Update Firefox to the latest release