Several extensions from 2013 and earlier stopped working in Firefox 40 because they used the old "widget" module and that module is no longer present in Firefox 40. Developers had a year to update their extensions, but I think this developer might have shut down; they let their registration for the teameuler.com expire.

I opened up the extension and it seems even though it calls on the widget module it doesn't need it for anything. I edited out that one line from the main.js file, and added a note that the extension was hacked to the title, and uploaded it here:

Edit: Please see the next post for an updated version of the extension.

You can download that, drag the xpi onto an open Firefox tab, and it should replace the version you have.

Two comments about that:

(1) Do you trust me?

(2) Since this version is not signed, it will stop working in Firefox 41.

Since this extension is open source software, a motivated person can submit a fixed version to the Add-ons site. I don't have any experience with that myself.

Hi, we've released v1.0.0 of the addon which fixes your issue, has cleaner code, and slightly cleaner design. Let us know if you have any more problems!

https://addons.mozilla.org/en-us/firefox/addon/hush-private-bookmarking/versions/?page=1#version-1.0.0