Some of those fixes are located in the add-on itself, some had to be made in the companion application (the external application that is to be installed along with the extension in order to provide the necessary capabilities removed from Firefox Quantum v57). Video DownloadHelper 7.3.1 brings a number of improvements and fixes that have been requested or reported by our users. Source code released under Custom License
There should be one or two simple-streamed variants (companion app is optional) and one to many ADP options (audio and video streams are transparently downloaded and aggregated locally, this requires the use of the companion app). When browsing a Facebook timeline or opening a Facebook video post, Video DownloadHelper offers several variants for each detected video. Video DownloadHelper 7.3.4 now includes support for downloading Facebook videos !