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There is no need for an extension! All you have to do is:
- Type about:config in the address bar. Then, if needed click the “I’ll be careful, I promise!” button.
- Next, in the filter box – type in: tabs.insertRelatedAfterCurrent

- Right-click that about:config entry, and select “Toggle”
via: http://www.firefoxfacts.com/2010/01/22/firefox-3-6-fix-open-new-tabs-in-far-right/