Extensions hosted at addons.mozilla.org are free. There are however a small number of extension where you can get a paid for version as it may give you more features or such. For example Video Video DownloadHelper is a popular free extension however the author(s) do provide a premium version for a price which gives you these features Download any videos as much as you want, no limits. Convert to your favourite format (MP4, Mkv, WebM…), Download videos as audio, No QR code in generated videos.