Starting with Firefox 3.6, the default behavior when opening a link on a new tab is to place it to the right of the currently active tab, whereas previously it would open at the far right. Most people aren't terribly bothered by this, nevertheless, if you open a lot of new tabs which you intend to read sequentially, then maybe you feel like the old behavior suits you better.
Luckily, Firefox is extremely flexible, allowing you to customize pretty much any setting through the about:config interface. Thus, in order to revert to the old setting which places new tabs at the far right follow these simple steps:
1. Type about:config into the location bar.
2. Click the I’ll be careful, I promise! button to proceed.
3. Type browser.tabs.insertRelatedAfterCurrent into the filter text field.
4. Double click browser.tabs.insertRelatedAfterCurrent to change its value to false.
5. Close the tab.