before/after*

You can modify these prefs on the about:config page to disable the full screen fade in and fade out. Leave out the quotes around the value (two zeros separated by a space). Note that this is the default on Linux (GTK).

• full-screen-api.transition-duration.enter = "0 0"
• full-screen-api.transition-duration.leave = "0 0"

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

I made the parameters that you gave true. But nothing changed.

The prefs I mentioned are String prefs and consist of two numbers zero (0) separated by a space: "0 0".