Be careful with old versions!
These versions are displayed for reference and testing purposes. You should always use the latest version of an add-on.
Actually Mozilla did find a way to break the UI but I didn't notice at first. Silly me, never underestimate the clever and innovative ways Mozilla will find to annoy users :-)
Tabs open in the correct place in FF4 now, i.e. to the right of the current tab. Externally clicked links open at the far right.
I recommend "Firefox 4 UI Fixer", "Switch to Tab no more" and "Custom Tab Width" to make the FF4 interface usable again. They really have outdone themselves this time, managing to move just about every element you look at regularly so that you end up scanning around looking for vital information.
FF 3.6 now opens tabs relative. I'm glad the Moz devs finally saw the light :-) All this extension does now is re-enabled the browser.link.open_external preference.