Rated 5 out of 5 stars

It looks like, sadly, Wikilook's development has stopped. And recent changes in Firefox mean that Wikilook is no longer working as it used to.
Fortunately, user Alan Stewart has posted a working fix at Wikilook's Support page (http://groups.google.com/group/wikilook).
In case you don't have access to that Support page, I'm copying the fix here so more people have access to it:

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In the file chrome/wikilook/content/wikilook.js line 4746 is not valid syntax in the newer javascript engine:

globalPackageWL.WikiLook_Overlay.PMaker.WLWTajaxCallbackImpl(res.expandtemplates.*);

Changing that line to an alternate equivalent syntax:

globalPackageWL.WikiLook_Overlay.PMaker.WLWTajaxCallbackImpl(res.expandtemplates['*']);

works for me - all of wikilook is functional with ff21
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All credit for this fix should go to Alan Stewart, I'm just copying his solution here.

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