Why do you want to hide this window?

It provides useful information about the page and gives access to the cookies of a specific tab that you can use in case of issues via the Control Center 'i' icon (More Information).

The Page Info window has this chrome URI:

• chrome://browser/content/pageinfo/pageInfo.xul

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

@-moz-document url-prefix(chrome://browser/content/pageinfo/pageInfo.xul){
 *{display: none !important}
}

Not that userChrome.css can be bypassed by starting Firefox in Safe Mode (hold down the Shift key while starting Firefox).

Hello cor-el, I have added the content to disable page info window but when I start Firefox it does not change anything, I have written it well is not it? I attach screenshots. One last question where you see chrome URI of each of the elements? I await your news. Thank you very much for the speed.

sorry for my mistake when I put the code in the user chrome.css worked perfectly and found this page to see all http://kb.mozillazine.org/Dev_:_Firefox_Chrome_URLs Thank you.

You're welcome.

Note that the mozillaZine knowledge base page you linked to is rather old (last updated in 2009), so a lot of those chrome URI no longer exist is current releases. Page Info is one of them that hasn't changed over the years, so this chrome URI is still valid.