Install the mFull add-on and see if it offers what you need.

It's not the same as hitting the full screen button.... close but no....

Why would you want to hit F11 if you want Firefox to start in fulll screen mode? ;)

The issue is that Firefox does not (or no longer) have a native way (command line option or other) to launch in fullscreen mode. If I install the add-on above and enable its "Always start in full screen" option, it does what it says, so it starts in fullscreen mode and toggling with F11 is still possible.

Other add-ons capable of doing the same like Autohide (which was capable of remembering the normal or full screen state at restart) and Full Fullscreen have been discontinued, but there may be more. I may have misunderstood your wish though.

Firefox no longer stores the Maximized and Full Screen mode setting in xulstore.json ("sizemode":"normal"), so it isn't possible to start Firefox in Full Screen mode without using an extension.

I have the opposite problem. I did not know there was a full screen mode until just now, when I must have hit some sequence of buttons by accident, and suddenly all the toolbars and menu bar went away. I'd like to prevent that from ever happening again. How can I completely disable full screen mode from ever activating?