I think it is used to check the coding but when I tried it didn't respond. I mean it doesn't give any response and showing the data of current website.
F12 opens the built in Firefox Developer Tools in recent versions of Firefox. Press F12 again to close it. There is an extension to turn off the shortcut: https://addons.mozilla.org/en-US/firefox/addon/disable-f12-shortcut/
F12 is not bound to anything in Firefox - see Keyboard shortcuts. You are probably thinking of the Firebug addon which opens with that key.
The built-in Page Inspector in Firefox (v10 onwards) is accessed with Ctrl+Shift+I and then Alt+M.
The f12 is a shortcut to Open up firebug in firefox. To open up firebug you must firstly have it installed
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