How to make Firefox 57 to always open new tab from address bar?

Firefox 60 actually has flag browser.urlbar.openintab in about:config. Setting it to true accomplishes what you want. Tried and true.

The only working solution I've found (currently) is an Add-on called "New Tab from Location Bar" by Piro.

The solution by andy_m didn't work for me.

Workarounds:

• Press and hold Alt when pressing Enter for the completed URL.
• Press and hold Ctrl while clicking on a link, or use middle-mouse button.

It would be simpler if these were the same keys, if there are Firefox developers reading this.

As a long-time TabMix Plus user, I really miss this and other features, such as shifting focus to a tab on mouse-hover. Until Firefox WebExtensions is more mature, apparently, that capability is gone. Sic transit gloria mundi.