Apparently the new version is now a beta, however I still want to explain what happened.
The add-on hasn't been hijacked or whatever. The developer just updated it to comply with the new WebExtensions API mozilla is trying to push out. However, this API bears limitations.
These include not being able to edit or even select text in the address bar, and not being able to access local files for "security reasons".
If you'd take a look at the developer's page, it explains what happened.
This surely won't be the only add-on to suffer the limitations of the coming WebExtensions API, and mozilla is working with a few add-on developers to open up the API to allow for more advanced features, but this one here is probably too niche for that to happen.