open about:config and disable browser.urlbar.doubleClickSelectsAll -> If true, select the contents of the Location Bar when double-clicking anywhere inside it.
open about:config and disable ui.key.menuAccessKeyFocuses -> If false, the Alt key doesn't show the menubar on Windows.
open about:config and disable browser.download.useDownloadDir -> Ask where to save each file before downloading
open about:config and enable browser.urlbar.update2.engineAliasRefresh -> Custom Search engine: from https://superuser.com/a/1756774