Currently broken - "Wiktionary and Google Translate" is a good alternative Rated 1 out of 5 stars

Sadly, Wikilook is broken again. Maybe someone can make it work again but, considering that the last update was over four years ago, I don't hold much hope.
I can confirm what kolika33 mentioned: Wikilook 2.7.0 conflicts with Adblock Plus. When Wikilook is active, Adblock Plus can't save custom filters. Strange, but true.
Meanwhile, I have found a good alternative for Wikilook. It's not 100% the same, but close enough, and in some respects even better. It's "Wiktionary and Google Translate" (https://addons.mozilla.org/en-US/firefox/addon/google-dictionary-and-google-t/). Try it if you're looking for a Wikilook replacement.

Bug fix for Firefox 20+ 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.

Rated 5 out of 5 stars

Great! It's working again (v. 2.2.1). Thanks to Vitaly, the developer, (or whoever updated it) for fixing it.

This review is for a previous version of the add-on (2.2.1). 

Thank you!

Thank you for support! Hope you like new smart search feature.

Rated 5 out of 5 stars

Wow! Excellent add-on. Thank you to the developer. May I add that I managed to change the green background color with a little hacking, but it could be implemented as an option in future releases.

This review is for a previous version of the add-on (1.3.4).