Is there a way to open up a URL in a background tab in Firefox?

Type ⌥ (Option)+Return to open a URL from the address bar in a new tab in Firefox within macOS.

In Windows... Rather than ctrl+enter, you could ctrl+click on the first item that shows up (in my case - whatever I typed in the search bar - Search with Google for search items or ... - Visit for URLs) for it to open in a new foreground tab, or ctrl+shift+click for it to open in a background tab. I don't have a Mac to test on, but there should be an equivalent so maybe play around with keys and clicks until you figure it out?

Probably not, but for Ctrl+click to link it works, if you in

Options, General, Tabs

uncheck the option for bringing the tab with new opened URL into foreground.

ALT-CTRL-SHIFT-ENTER will open a URL from the address bar in a new background tab.

see: https://bugzilla.mozilla.org/show_bug.cgi?id=1506203