Please note that browser.tabs.insertRelatedAfterCurrent preference only affects opening of related tabs - those which are opened by right clicking on a link and selecting Open Link in New Tab. All blank tabs would open at far right because they are not related to the current tab in any way. This is the default Firefox behavior.

If you wish to open the blank tabs also next to the current tab, you can install an add-on like Tab Control.

thanks dynamite, Tab Control is just what I was looking for, ....and it works fine.

But this... "opening of related tabs - those which are opened by right clicking on a link and selecting Open Link in New Tab" ...does not work. A related tab was allways opened on the far right, the state of the boolean did not change anything.