This add-on was build using the add-on-sdk, which was the preferred way of building add-ons at the time. Now mozilla is trying to push everyone to their new web-extension framework, because they don't want to support the classical methods any more. Therefore they are displaying this warning for all extensions that are not web-extensions.
This addon does nothing more than what is described, especially not accessing or collecting any data. I am not sure yet, if I will port this extension to the web-extension framework.
This is very useful. Works well. I've been wanting this functionality for quite some time. This is great for renaming streams before downloading with Video Download Helper, as oftentimes the stream will have a different file name than the descriptive link to the stream and copy/paste with a link can certainly be difficult at times. Perfect!