Also uncheck "Open new windows in a new tab instead" in Options in GUI or set browser.link.open_newwindow to 2 instead of 3 (it's the same thing) to make pop ups open in new windows instead of tabs.
There might be an extension to remap the Ctrl/Shift shortcut or abandon tabs altogether but I don't know of any.
Googling turns up results like https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus/ but it's of course not compatible with Quantum.
I've been using https://addons.mozilla.org/en-US/firefox/addon/hide-tab-bar-... but it seems this isn't going to make it into the Firefox Quantum era. :(
Mozilla is making noises about disabling this eventually, but it works for now.
https://superuser.com/questions/1261660/firefox-quantum-ver-...
works for me: https://i.imgur.com/ZMmJlZT.png
I don't have any implementations I can point to, though. And a brief browse of about:config didn't yield anything interesting.