There is no need to highlight anything if you are using Shift + mouse move. This way you could look up words over hyperlinks. Or, if you interested in looking up lots of words, you hold down shift and move mouse around, positioning it over words one by one.
You can redefine parser for mouse gesture to make it look up Wikipedia. Or, if you need to look up both, then there is a workaround in WikiLook 2.5.8.
http://downloads.mozdev.org/wikilook/wikilook.xpi You can use context menu, so you highlight something with mouse, right click, and select WikiLook from drop down menu. But dragging would not work on some web sites with context menu div. Need to look closely at DOM...
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