Dissable exit full screen message

I have an experiment where participants need to use the mouse and is very likely they will put it at the top of the screen at some point, so it will trigger the message 'url.com is now full screen. Exit Full Screen (Esc)" in Firefox or “Press Esc to exit full screen” in Chrome. Is it possible to disable this message?

To be more precise, I’m OK with the message the first time the window goes to full screen, it’s only the ones after that that appear when placing the cursor at the top of the screen the ones I want to get rid of.

Here someone suggested:

full-screen-api.warning.delay = -1
full-screen-api.warning.timeout = -1

but I’m guessing this is local firefox configuration (?). Anyway, I put it within a JS snippet at the beginning of the experiment but it threw a SyntaxError: invalid assignment left-hand side.