Mozilla have changed the way that new tabs operate in Firefox 3.6. When opening a new tab from a link on a web-page, the new version of Firefox will place it to the right of the current tab, rather than at the far right of the list as it did in previous versions. To revert to the old functionality type about:config in the address bar and then click the "I'll be careful, I promise!" button if it appears.
In the filter type tabs.insertRelatedAfterCurrent and then right-click on the entry and click "Toggle".
