in your address bar, go to "about:config"
the do a search for "browser.tabs.showSingleWindowModePrefs" (if it's not there, you have to right click --> New --> Boolean, and then add it)
set to "True"
Restart Firefox
Go to Tools --> Options -> Advanced --> look at "Tabbed Browsing" options
Should now have an option for "Force links that open new windows to open in:
same tab/window as the link
a new tab
You should be able to take it from there.