Do-not-track is now a three state setting and is controlled by two prefs: one to enable/disable the DNT header and another to determine its value.

• privacy.donottrackheader.enabled (false)
• privacy.donottrackheader.value (1)

Your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts.

The user.js file is only present if you or other software has created this file and normally it wouldn't be there. You can check its content with a plain text editor (right-click: Open with) if you didn't create this file yourself.

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

• http://kb.mozillazine.org/Preferences_not_saved

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

Didn't work, even with multiple tries. Fix Firefox...

How do these prefs show on the about:config page?

Did you try to delete the prefs.js file and possible user.js file to reset all prefs?

Did you check for issues caused by extensions?

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Can't you change this setting at all or does it only revert when you close close and restart Firefox or when you reboot the computer?