In Firefox 3.6, when you want to open a link in a new tab, it will now open the link next to the tab you are currently in. This feature was not available in previous Firefox version and can be disable by following these simple steps:
1. In the address bar, type about:config and click I'll be careful, I promise!.
2. In the Filter box type browser.tabs.insertRelatedAfterCurrent
3. Double click on the browser.tabs.insertRelatedAfterCurrent filter result to change its Value to false. And you're done!
As always, please click on the image to enlarge!