Unfortunately, middle click, right click + open new tab, and Ctrl-click don't always work - if the link also triggers something on the site or the site restricts alternate clicks then; middle, right, and Ctrl-clicks don't work.
Posted June 3, 2018
Gleam is one main site that springs to mind - middle and right clicking are disabled.
@Deranox: You can't right now. Rather, use one of the other options to open a background tab. 1) select "open link in new background tab" from the context menu (second choice), 2) use middle-click on the link with your mouse; 3) use Ctrl+leftclick on the link with your mouse, 4) designate a mouse gesture to open a background tab when you perform it over a link.