Hello, Can you tell us which addon you use?

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.

You could also download the video as is, and use a free converter program to get what you want.

This one : https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/?src=cb-dl-mostpopular

I've downloaded Wondershare but while downloading it stops and there's a message says that Failed obtaining video info !!!

Download has worked but the problem that the video itself was in low quility and I was downloading in HQ. Then, I used Wondershare to convert it to MP3.

Thank you very much for your cooperation :)

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for download and convert.

Separately !