The extension needs to be reviewed by Mozilla and signed unfortunately. 80% get done within 5 days but unclear if this will be the case here. Meanwhile you can use the bookmarklets at https://ctxt.io/faq but those won't work on CORS/CSP restricted sites.
Although, at this moment, my Firefox (52.0.1 on GNU/Linux) says the extension cannot be installed because it is faulty what ever that might mean.