In Firefox 3.6, new tabs created from opening links are positioned next to the current tab. This behavior makes it easier than before to keep similar tabs close together.
If you would like to use the old behavior from Firefox 3.5 and earlier, in which new tabs open at the end of the tab strip, install the New Tabs at the End extension.
If you ever want to change it back, see Use extensions to add features to Firefox.
For more information about tabbed browsing, see Use tabs to organize lots of websites in a single window.