Since Firefox 3.6, new tabs open next to its parent. This is probably one of the features written most about after the new release. Or rather how to disable it, that is. I'll just add it to my notes, so I can perform the fix on every new install.
- Type about:config in the address bar.
- Type browser.tabs.insertRelatedAfterCurrent in the Filter bar.
- The the value to false, by double-clicking or right-clicking.