Grabby is a WebExtension that was created with the purpose of providing the same functionality that the FlashGot addon provided in legacy addon API and older browsers. Features:

Allows you to download files using external download managers.
Override's Firefox's default download dialog and lets you choose which download manager you want to download your file with.
Allows you to download videos from video sharing sites.
Ability to "grab" links in a page and filter them for downloading with your preferred download manager.
Enhanced video download features for YouTube such as downloading playlists and downloading audio

In order to use this addon you need to download and install the a toolkit too. The latest version of the toolkit is available here: https://github.com/pouriap/Grabby-Toolkit/releases/latest The toolkit includes the native application which is required for WebExtension addons to be able to communicate with other programs (such as download managers) and other tools required for the addon's functionality such as downloading videos. For more information about the addon and the toolkit visit Grabby's github page: https://github.com/pouriap/Grabby