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

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

Add code to the <b>userChrome.css</b> file below the default @namespace line. *http://kb.mozillazine.org/userChrome.css ---- <pre><nowiki>@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */ #fullscr-toggler { display:none!important; } </nowiki></pre> ---- The customization files userChrome.css (user interface) and userContent.css (websites) are located in the <b>chrome</b> folder in the Firefox profile folder. *http://kb.mozillazine.org/Editing_configuration You can use this button to go to the current Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox *Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist *Use a plain text editor like Notepad to create a (new) userChrome.css file in the chrome folder (file name is case sensitive) *Paste the code in the userChrome.css file in the editor window *Make sure that the userChrome.css file starts with the default @namespace line *Make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css.<br>Otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.txt file