Not only Firefox but AFAIK most browsers -- at least on Windows -- have interpreted "target=new" as new tab, not window, for several years now. "Open in new tab" is also the first alternative on the right-click menu, before "Open in new window".

Also, BTW, Firefox has had a Tabs menu for years now; both vertical and hides itself away automatically. (Though perhaps you still can't save any vertical space with it: AFAICR it lives on the tab bar, so if you hide that you lose the menu too.)