You can set the pref browser.tabs.insertRelatedAfterCurrent to false on the about:config page.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold (user set).
• Preferences can be reset to the default via the right-click context menu if they are user set
• Preferences can be changed via the right-click context menu: Modify (String or Integer) or Toggle (Boolean)

That's it! Brilliant, thank you. Why FF have to make it so difficult I don't know......

You're welcome

See also:

• https://support.mozilla.com/kb/common-questions-after-updating-firefox
• http://www.mozilla.com/firefox/features.html