Please see GIF under This is the website where I see this issue. I want stay on current tab. Note I never ticked "When you open a link in a new tab, switch to it immediately".
The HTML of the website is opening the link with the option of target=_blank which causes it to open in a new window or tab (according to browser settings).
You do not need such a complicated website to demonstrate this behavior. This minimal Tryit example is already enough to demonstrate it.
In Firefox this behavior can be disabled by entering
in the address bar, then setting
For more details see the Mozilla Support article
Can I force a link to open in a non-active tab, instead of changing to that tab?