Regarding Firefox 57: I'm no longer using Firefox as my primary web browser, therefore I'm not going to convert my add-ons to webextensions. Link to the source code is available, if one needs to fork the project.
Magrent enables you to convert magnet links and torrent hashes to torrent files. It uses torrent storage services like Torrage.com for downloading torrent files.
This add-on is useful when you have the magnet URI and want the torrent file or when your are using sites like The Pirate Bay which only provide the magnet links.
Magnet/Hash context menu items
These items appear when you right-click on a magnet link or select a torrent info hash and then right-click.
"Download torrent file" will download the torrent from the default torrent source(if the torrent is available). You can change the default torrent source from Preferences/Options.
"Open in Magrent panel" opens the magnet URI or hash in the panel and shows its download links.
Magrent button and panel
Clicking on the add-on's button will open a panel for converting magnet URIs and hashes to torrent files. It checks your clipboard content and when it recognizes a magnet URI or a hash, it will convert them automatically and show the download links.
Download links are from iTorrents.org, btcache.me, Torrasave.top and TheTorrent.org.