If you cannot access the settings, the implication is that your PC has some administrator level controls in place. Either your current login does not have local administrator privileges or your PC is part of an enterprise Domain which applies proxy restrictions on you. If this is the case, it has been done badly because the appropriate restrictions haven't also been applied to Firefox. Quite probably, in this case, the issue is because you are not connected to your enterprise network. You will need to contact your IT department to fix their proxy settings.