You can disable this feature via code in userChrome.css.

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

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

#fullscr-toggler {display:none!important}

It is not that difficult to create userChrome.css 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 chrome in this folder (name is all lowercase). In the chrome folder you need to create a plain text file with the name userChrome.css (name is case sensitive). In this userChrome.css text file you paste the posted text.

In Windows saving the file is usually the only time things get more complicated because Windows can silently add a .txt file extension and you end up with a file named userChrome.css.txt. To avoid this you need to make sure to select "All files" in the dialog to save the file in the text editor using "Save File as".

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 about:config 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

Thanks to Cor-el ! I just use this browser 2 days ago because my browser game have some error with google chrome, and it's make the game lag very hard. I hope I will have some good time with this browser !

Thanks.

Thank you very much! A friend of mine, who's working on exhibitions, asked me to build a documentation about the current exhibition. Visitors are allowed to use a screen and a mouse to navigate into the documentation pages (not by keyboard). These pages are builded like a website (html, css and javascript), and the result is shown at fullscreen, but visitors are not allowed to reach the tabs or close buttons of Firefox of course, and as they don't have a keyboard, your solution works perfect to me. Thank's a lot !

Note that Firefox also comes with a kiosk mode.

• https://support.mozilla.org/en-US/kb/firefox-enterprise-kiosk-mode