"Couldn't load XPCOM.". Good morning to you too, Firefox 60.

Ready2Search

Create custom search plugins for Firefox. Neat!

Content Security Policy

As made clear by the CSP spec, browser bookmarklets shouldn't be affected by CSP. [..]

But, none of the browsers get this correct. All cause CSP violations and prevent the bookmarklet from functioning.

Though its highly discouraged, you can disable CSP in Firefox as a temporary workaround. Open up about:config and set security.csp.enable to false.

Finally I can bookmark github.com pages though my Firefox bookmarklets again. Wasn't aware that CSP was the culprit...