This is not quite universal (the same issue occurs with Falkon browser).
However, 'normally' we use the middle-click to signify that we want a link opened in the background without us losing focus - usually when we want to open a couple of items.
In the URL bar, we might start typing and see several promising suggestions, but if we middle-click them we are taken straight to them in a new tab (losing focus).
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I disagree. I really like that it immediately opens in a new tab and gives it focus. I have been using it for a long time and I prefer the way Firefox handles this. Changing it now would alienate all the users who are currently using this feature.