How remove the "Firefox Developer Tools" from Firefox 28.0?
Is there a way or script or option in about:config to remove them permanently?
You can't remove them, because they are integrated into Firefox. Though various tools have a corresponding preference in about:config to enable/disable them.
Just search for devtools.*.enabled there and set all of the appearing options to false.
See also the thread about this in the support.mozilla.org forum.
Note: As long as the DevTools are not opened they don't use any memory.