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

#fullscr-toggler { display: none !important; }

You can add code to the <b>userChrome.css</b> file below the default @namespace line. <hr> <pre><nowiki>@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */ #fullscr-toggler { display: none !important; } </nowiki></pre> <hr> It is not that difficult to create <b>userChrome.css</b> if you have never used it. The first step is to open the "Help -> Troubleshooting Information" page and find the button to access the profile folder. *https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox You can find this button under the "Application Basics" section as "Profile Folder -> Open Folder". If you click this button then you open the profile folder in the Windows File Explorer. You need to create a folder with the name <u>chrome</u> in this folder (name is all lowercase). In the <b>chrome</b> folder you need to create a plain text file with the name <b>userChrome.css</b> (name is case sensitive). In this userChrome.css text file you paste the text posted. You need to close (Quit/Exit) and restart Firefox when you create or modify the userChrome.css file. See also: *https://www.userchrome.org/what-is-userchrome-css.html *https://www.userchrome.org/how-create-userchrome-css.html In Firefox 69 and later you need to set this pref to true on the <b>about:config</b> page to enable userChrome.css and userContent.css in the chrome folder. *toolkit.legacyUserProfileCustomizations.stylesheets = true See: *https://www.userchrome.org/firefox-changes-userchrome-css.html