Are you on a computer or network that's not yours? For example, is this a work computer or a school computer?

If you are, then it's possible that the system administrator for your IT department has set and locked that setting. If you are not the owner of the computer, you need to contact your IT department for more information and assistance.

If this is your computer, you will want to check for a firefox.cfg file in your Firefox installation folder. This would be what is likely locking that setting.

Hope this helps.

In Firefox 63+ you can check the about:policies#active page to see whether policies are active.

You can inspect the Mozilla and Firefox keys with the Windows Registry Editor in HKEY_LOCAL_MACHINE and possibly in HKEY_CURRENT_USER with the Windows Registry Editor to see whether GPO policy rules are active.

• HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\Firefox\

Note that the mere presence of the "MozillaFirefox" key is sufficient to make Firefox display this notification, so if you have the Firefox key then remove it and only leave the Mozilla key or remove this key as well if it is empty.

• HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\Firefox\ =>
  HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\