If you’ve updated to the latest versions of Firefox (version 43 and later) and you receive the message “Plugin could not be verified for use in Firefox and has been disabled“, you will need to update the plugin to the latest version.
Older versions of Firefox allowed plugins to install without warnings if they were signed from the developer (in this case codebender). This was changed to require extra signing through Mozilla, the non-profit organization that is behind Firefox. For a couple of versions before 43, plugins not signed through Mozilla were allowed, although a warning was displayed in the add-ons page.
From Firefox 43 the default is to disable the plugins that are not signed with the new system. While the user can override this, version 44 will most likely not allow plugins that are not signed with the new system. If you’d like to override this for codebender, or any other extension (although for codebender it is not really needed), type about:config in your location bar, and change setting xpinstall.signatures.required to false.
If you haven’t used codebender with Firefox for a while and you update Firefox before visiting codebender you won’t see the option to upgrade your plugin. Since the browser will disable our plugin before the page loads, it will appear as not installed and not as needing updating. Following the link to install the plugin again will update it nevertheless.