Yes, you have to install a java script, and you have to install ffmpeg. and you have to let the add on 'read' any webpage you are using.

That's how software works. j

If you want to mux and demux audio and video, you must install an app that can perform the task, and you must install a go between between the web browser (the front end) and the a/v editing software that performs that function at the back end. (ffmpeg - which is open source and been around for a long, long time).

So the js.node is the link between firefox and ffmpeg.

And if you want to download a file from the web and then demux it, or two files, and mux them, of course the addon *must* scan the whole page to locate the elements you want it to find. Is this a security risk? Of course it is - any time you install any app (even a firewall or antivirus), be definition, they must scan everything in order to do their jobs.

I've used this and other apps from the developer for a long time. They do what they say. Any software you choose to download is a question of trust. At least with this one, they step by step walk you through everything you are doing, so you know what's happening.

Regarding the combine function, you *must* select both audio and video file at the same time using windows explorer and drag them both together to the app window

js.node and ffmpeg are also open source apps, and updated for numerous reasons over time, and not made, not owned, by this add-on maker, so linking to someone else's software is required.