There is a problem with this extension that needs to be addressed by its author via an update.

The Browser Console (Firefox/Tools > Web Developer) shows an error message: href.quote() is not a function

The extension uses the quote() function to enclose a string in quotes.

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/quote

Obsolete since Gecko 37 (Firefox 37 / Thunderbird 37 / SeaMonkey 2.34) This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

The quote() function is used in overlay.js and needs to be replaced:

var s = document.createProcessingInstruction('xml-stylesheet', 'href=' + href.quote())

var s = document.createProcessingInstruction('xml-stylesheet', 'href="' + href+'"');

Editing this file make the signing invalid, so you need to remove the META-INF folder. This make the button appear fore me.

I created a bug report about this, and others;

Please do not comment in bug reports. Bug 1155413 - Many icons missing from Add-on Customize Manager https://bugzilla.mozilla.org/show_bug.cgi?id=1155413

I have sent messages to the (so-called) support e-mail, but never heard back.

I've checked the Webutation extension and this extension is broken because it uses a function (quote() to quote a string) that is no longer supported in Firefox.

This is not a bug in Firefox, but a bug in the extension and thus needs to be fixed by the creator of the extension. You can point the creator of the extension to this thread if you like.

I don't know about the other extensions, but they might be broken as well and you would have to check the Browser Console (Firefox/Tools > Web Developer) for error messages.

SUCCESS ! ! SUCCESS ! ! SUCCESS ! !

Thank you, cor-el, Oh Mighty Mozillion.

I downloaded a new Webutation add-on without installing it. I opened the XPI in WinZip to work on it. I removed the META-INF folder as you said above. I then edited the content/Overlay.js file. I replaced ('xml-stylesheet', 'href=' + href.quote()) with ('xml-stylesheet', 'href="' + href+'"');

It looks like Webutation is working properly.

Many, many thanks, Oh Mighty One.