I have collected the solutions to make this great add-on working... the two explanations describe the same solution, so you only need to follow one of them.
With FF 43, you need to change the options in the browser to make your modified "unsigned" add-on working, and after FF 44, it will not work!
"Firefox 43: Firefox will have a preference that allows signature enforcement to be disabled (xpinstall.signatures.required in about:config). Firefox 44: Release and Beta versions of Firefox will not allow unsigned extensions to be installed, with no override."
With ff21, I get a syntax error in the error console, that stops wikilook 2.7.0 from finishing loading.
works for me - all of wikilook is functional with ff21.
Contrary to what has been mentioned, I don't see where wikilook is using the deprecated E2X syntax. In any case, that wouldn't require a complete re-write.
To get the full file name, you have to know the path to Firefox's profile. Do this:
Enter this special URL in the address line: about:support You'll get a page with title "Troubleshooting Information".
In the top third you find a table with "Application Basics"; it contains a row "Profile folder" with a button labeled "Show Folder". Press the button. An Explorer window will open showing the location and the contents of your profile. Now you'll have to navigate through several levels of sub-directories... Open folders extensions, then wikilook@testpilot. You are now on the level Alan was when he said "In the file chrome/wikilook/content/wikilook.js line 4746 is not valid syntax". Dive down through folders chrome -> wikilook -> content. Now you can see file wikilook.js. Select it (don't execute). Terminate Firefox before you edit this file.
Editing wikilook.js may be a problem for the inexperienced user. There are 2 options:
Use a good editor (as I did) which can handle Unix line-breaks (Windows' Notepad cannot, see below). You can drag & drop the file into the editor's window (if that doesn't work, you now know the full path of the file anyway). Then go to line 4746 (or do the search shown below) and apply Alan's change. If you have no other editor than Windows' Notepad, do the following: Right-click at wikilook.js and select edit from the menu (not open, don't double-click). This will load the file into the Notepad editor. Bad surprise: the whole text is shown as one huge continuous line! But you're not lost. Just search (Ctrl+F) for res.expandtemplates.* and apply the change. (res.expandtemplates.* => res.expandtemplates['*'])
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.
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)