Type this in address bar :-   about:robots

You’ll see the following message in the browser:

Welcome Humans!

We have come to visit you in peace and with goodwill!

* Robots may not injure a human being or, through inaction,
allow a human being to come to harm.

*Robots have seen things you people wouldn't believe.

*Robots are Your Plastic Pal Who's Fun To Be With.

*Robots have shiny metal posteriors which should
not be bitten.

And they have a plan.

And when you type :- about:Mozilla

You’ll get this message written in white text on red background.

Mammon slept. And the beast reborn spread over the earth
 and its numbers grew legion. And they proclaimed the times
 and sacrificed crops unto the fire, with the cunning
of foxes. And they built a new world in their own
 image as promised  by the sacred words, and spoke
 of the beast with their children. Mammon awoke,
 and lo! it was naught but a follower.

from The Book of Mozilla, 11:9
(10th Edition)

Another method is to look for prefs.js file in firefox profile directory. Before you edit that file, exit the firefox. Then open the file with text editor. Search for the entry you want to delete. When you find it, delete the line. Save the changes made to file, exit the text editor. Now you can start the firefox, and check the about:config entries.