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.

This user has 3 previous reviews of this add-on.

Try rajeshWikiFinderInWiki Rated 1 out of 5 stars

Wikilook doesn't work with encrypted Wikipedia.
Tey rajeshWikiFinderInWiki, not as good as Wikilook, but one part.
Please note other substitutes for wikilook you find.

This user has a previous review of this add-on.

Conflicts with Adblock Plus ! Rated 1 out of 5 stars

If add-on WikiLook 2.7.0 is on, Adblock Plus don't save the filters. I can modify it, but after Firefox (37.0.1) restarts all changes are lost.

Absolutely no effect whatsoever. Doesn't work. Rated 1 out of 5 stars

It does not work. Too bad, it has lots of great reviews from the past, but it appears to be broken now.

It is really cool Rated 5 out of 5 stars

OneTimePad yes it is cool. I love it as well.

This user has 4 previous reviews of this add-on.

Rated 5 out of 5 stars

I simly love it. It has room for improvements, but all in all it is soooooo cool! :)

Rated 5 out of 5 stars

Не фурычит чёта... ставлю 5 за красивые скриншоты, надеюсь на исправление.

Rated 1 out of 5 stars

Used to love this add-on. Now it apparently is broken. Select, move slightly, and hover. No. That was in fact broken a long time ago. But now the Shift function also does not work. Add-on removed.

This user has a previous review of this add-on.

Terrible extension. Does not work Rated 2 out of 5 stars

Calls itself open source, but has no way to access the code.

Have never seen it work once, even through the context menu.

Rated 3 out of 5 stars

Does not work, please fix that! I am under Firefox 31...

.....Thanks

Ne fonctionne pas Rated 1 out of 5 stars

J'ai beau essayer de mettre la souris sur un mot, de faire shift ou quoique que ce soit indiqué dans la présentation : rien ne se passe.
Extension inutile.
Dommage.

I made some sort of replacement to WikiLook: https://addons.mozilla.org/en-US/firefox/addon/otheraddon/ . Rated 1 out of 5 stars

I made some sort of replacement to WikiLook: https://addons.mozilla.org/en-US/firefox/addon/otheraddon/ . If you select a word and press Ctrl+Shift+1 it will open a Wikipedia layover and if you press Ctrl+Shift+2 it will open Wiktionary; pressing Ctrl+Shift+` or pressing the X button closes the layover. It also adds useful search buttons to the taskbar because I didn't like having to select from the drop down menu every time I wanted to use a different search engine. Let me know if you see any gaping bugs.
(Note, my rating is because it no longer works, when it did, it was great)

Doesn't work Rated 1 out of 5 stars

It's still a raw addon.
Sometimes it doesn't work at all and I can't figure out why.
It seems to be it conflicts with other extensions.
Firefox 23.0.1.

Rated 3 out of 5 stars

Please update for FF22...This is an essential addon....

Rated 1 out of 5 stars

Doesn't work in Firefox 22.0

No development since 2011 Rated 1 out of 5 stars

Doesn't work with FF 20.0.1Nearly no development, otherwise it would be 5 stars!
You can try QuickWiki as alternative!!
https://addons.mozilla.org/de/firefox/addon/quickwiki

What happened? Rated 4 out of 5 stars

Awesome Add-on, but hasn't worked since I updated to Firefox 20.0.1.

Rated 3 out of 5 stars

No longer working with Firefox 20

This user has a previous review of this add-on.

Rated 1 out of 5 stars

I didn't know that I was dependent on this add on this much. I've came to realize that I do, after the last Firefox update because it's not working anymore.

Please fix it. TestPilot and Rasha, you guys were really helping people learn.

Rated 5 out of 5 stars

Everything has been awesome with WikiLook until today, when my Firefox auto-updated itself to version 20.0.1, and now I can't get it to work anymore :(

However, I give this add-on 5 stars out of hoping it is just a sort of compatibility issue that can hopefully be solved soon.