Check on the about:config page whether these prefs have their default value (i.e. are not bold).

• devtools.chrome.enabled (false)
• devtools.debugger.prompt (true)
• devtools.debugger.remote-enabled (false)

You can check if you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

If you have a user.js file then you can check its content with a text editor (right-click: "Open with"; do not double-click). 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. You will have to reset prefs set via user.js via the about:config page to restore the default.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

You can delete the user.js file if you didn't create this file yourself.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data