Firefox was driving me crazy- I open tabs most of the time with CTRL + click and even though the option to open the tab right away isn’t checked that’s what it does. Happily, I just found this note with a tip on making Firefox open tabs in the background:

In order to force all links that open in a new window to load in a background tab, type about:config in the URL bar and change browser.tabs.loadDivertedInBackground to true.