hide top bar in firefox

hide.md

ctrl + shift + j

(function(){ /*toggle toolbox*/
var doc = Services.wm.getMostRecentWindow("navigator:browser").window.document;
var nt = doc.getElementById("navigator-toolbox");
var ds = nt.style.getPropertyValue("display") == "none";
nt.style.setProperty("display",ds?"-moz-box":"none","important");}
)()

https://support.mozilla.org/bm/questions/1256393