At first it didn't work, but then I disabled "New Tab Override" extension, and now this extension works great!
(I subsequently replaced "New Tab Override" extension with "New Tab Homepage" and there is no conflict with this extension.)

As others mentioned, it would be nice to have this directly on the context-menu (not sub-menu thereof).

(UPDATE: Yay! Version 2.2 2018-01-16 put the "New Tab Beside" menu item directly on the tab context menu.)

More importantly, the keyboard shortcut Ctrl + Shift + 1 is difficult. I'd prefer overriding Ctrl + Tab. Otherwise, Ctrl + Shift + 9 so my fingers can stay on the keyboard home keys.

almost perfect. it adds a context menu and works. It would be nice if you could only add "new tab besides" without sub menu. (duplicate tab could be another extension or you could have the option to use it or not).

Thanks for this! In my opinion, it would be even better to make just one button in the context menu, "New Tab". For "Duplicate Tab", there are already other addons. And my personal preference would be to make it the first item in the context menu, like in Chrome. That way it would be much faster to use.

Although, the shortcut is very handy (thanks!) so maybe I'll just learn to use that.

Very handy! Saves me just a couple seconds of rearranging every time I open a new tab, but that time adds up.