To get firefox to change the behavior to divert links to a new site to a different tab and to open that tab in the background (and also open bookmarks in a foreground tab):
- open "about:config"
- set browser.link.open_newwindow to 3
- set browser.tabs.loadBookmarksInTabs to true.
- set browser.tabs.loadDivertedInBackground to TRUE
Info from: https://support.mozilla.org/en-US/questions/1184461
Also see this page: http://kb.mozillazine.org/About:config_entries#Browser.