Open links in background on Firefox

Recently, I had to open 20+ URLs that were stored in a Checkvist list. I noticed that control-clicking the links don’t open tabs in background with Firefox on Windows. Other links do open in background, so it’s just the links inside lists.

With Chrome, the links open in background. Can anybody confirm this Firefox-specific anomaly?

Reproduces for me (for Firefox on Mac), possibly caused by some of our code which opens links explicitly with javascript, overriding the default browser mechanism. Requires extra investigation.