Hello,

This extension does not record sound, it only records the screen/application selected.
It is using the standard screen capture API which does not support audio in Firefox for now https://developer.mozilla.org/en-US/docs/Web/API/Screen_Capture_API#browser_compatibility

When it will be implemented by Mozilla, you will be able to capture it either to capture your web pages sound, your whole sound system or the defined microphone. But I do not know when or if Mozilla will implement it.
https://developer.mozilla.org/en-US/docs/Web/API/Screen_Capture_API/Using_Screen_Capture#capturing_shared_audio

On my side I did not have this need, but I can open an Issue about it for when I have to time to work on it.

When you say: "only viewable on your own firefox", I suppose you are talking about the downloaded file. Its extension is webm, intended for Browsers to read it easily, but I think you can convert it to mp4 easily with an application or read it with some other application than a Browser.
I will add the possibility to choose the extension from a list when I have the time.

If you want you can open a pull request on the project with these features ready. Or open an Issue with details on what you may want from this application. I will try to work on this when I have some free time.
https://gitlab.com/bdelseny/screen-capture

Thank you for your comment. I will try to work on this addon soon.