You will have to contact the Developer of this Extension. Mozilla just provides a outlet (like store) for them to post what they make. https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus/ and fyi : https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly Oh...go to Help --> About and see if you will get updated to ver 56.0.1 if not it will get to you. Firefox Quantum ver 57 release date Nov 14th fyi

Please let us know if this solved your issue or if need further assistance.

Thanks. I'm aware that TMP is not likely to be upgraded, at least any time soon and I've been monitoring the author's web site for any updated news. So it doesn't really solve my issue, I'm afraid, but thank you for responding. What I really want to know is if, native to Firefox 57, there's maybe an option (accessible via about:config) which will do what I want, that is, to reiterate, to allow a URL typed into the address box to be opened in a new tab with just <enter> pressed

Try https://addons.mozilla.org/en-US/firefox/addon/new-tab-from-location-bar/

It's not perfect, but it works

I've tried this and it does seem to do what I want, thank you. Occasional hiccups seem to occur but on the whole it's what I needed. Thank you.