Among the changes in the official Firefox 3.6 release is a change to where new tabs open from clicked links—to the right of the tab they launched from. Liked it better the old way? Here's the quick fix.
The Mozilla Links blog points out a few about:config tweaks that Firefox 3.6 users might be intrigued by, especially if the new version handles things a bit differently than you're used to, and you don't want to spend time getting used to the new scheme. To set new tab behaviour back to its old ways—opening in the right-most slot—enter about:config into your address bar, click that you'll be careful, then enter browser.tabs.insertRelatedAfterCurrent into the search bar. Set that value to "False," and your new tabs will open where they used to.
For more Firefox 3.6 tweaks, visit Mozilla Links' post, which details tab preview behaviours in detail.
Firefox 3.6 tips and tweak [Mozilla Links]