Mozilla Firefox: New tab settings
  1. Enter about:config in the address bar.

  2. Search for browser.tabs.insertRelatedAfterCurrent

  3. The value true (default) means new a new tab will be inserted directly after/right to the original tab, instead at the end.

  4. In order to force all links that open in a new window to load in a background tab, set browser.tabs.loadDivertedInBackground to true.