Just tried an extension that sounds like it might have worked for my problem but it doesn't even work. Fullscreen Plus You're supposed to use Ctrl+Shift+F but nothing happens when I do that. Tried restarting Firefox. Seems to be no way of contacting the person that wrote it on GitHub. So irritating when they can't even show an email contact or have a messaging page. Should be called "Write and Hide". The author's website doesn't work: http://trishulgoel.com

Maybe someone else will take up the reins.

You can use code in userChrome.css to disable the fullscreen toggler

@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 text posted.

On Mac you can use the TextEdit utility to create the userChrome.css file as a plain text file.

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