We do not really discuss add-on comparisons here, but note all officially hosted add-on extensions hosted on addons.mozilla.org and available from the Firefox User Interface will have been reviewed and be safe, secure and virus free.

• Use Menu Button -> Add-ons -> [Search all Add-ons]
  (Or key in to the Addressbar about:addons)
• Example search (You may be able to do a better targeted search from your own machine)
  • With >5M users top of the list is Video DownloadHelper

N.B. There may be possible legal &/or copyright issues with downloading some material, but that is out of scope of this support forum discussion.

newmand said

What is a safe add-on to download youtube video and audio, something virus and malware free?

Many thanks for the prompt and informative reply, I found the required "add-on" many thanks!

You're welcome. Glad to have helped, and thanks for posting back.