Firefox 3.6 includes a new feature where new tab, when click on a hyperlink that targeted to open in new window or tab, middle-click a link, or right click a link and select “Open Link in New Tab”, will be inserted and opened right next after the current or existing active tab containing the link in the Tab bar. The behavior facilitate the related tabs, where tabs for origin web page and web pages that are open by following hyperlinks that span from origin page are grouped together.

Read the full thing at http://www.mydigitallife.info/2010/02/01/change-firefox-to-open-new-tab-at-far-right-end-of-tabbar-disable-insert-next-to-current-active-tab/