The default behavior for links opening in new tabs in Firefox 3.6 is relevant to the tab from which the link is being opened. This means instead of opening at the far right of the tab bar, new tabs are being opened to the right of the current tab. Some folks like this, most don’t. To get new tabs to open on the far right like they did prior to Firefox 3.6, this can be done with a simple about:config tweak:
- In a new tab type about:config in the address bar and press enter
- In the filter field type browser.tabs.insertRelatedAfterCurrent
- Double-click the entry to change it to false.
- Close the tab and restart Firefox
Source: Mozilla Links