First off, you need to download Firefox Web Browser app from the AppStore.
Open the Facebook app and Tap Share on any video on your feed or timeline, then Tap Copy Link.
Go to FBDOWN.net, and Paste the Video Link before tapping Download.
Tap and Hold on any video format available (HD or SD). Then Tap Download Link and confirm the download.
Note 1:You can find your downloaded videos under the ‘Downloads’ section in Firefox menu.
Note 2:If Firefox isn’t available in your AppStore, you can use any other Web Browser that allows downloading (other than Chrome and Safari).