Review for FlashGot Mass Downloader by Detergent
Rated 4 out of 5 stars
Other download add-ons are often confusing or require the user to buy a pro-version to get the same functionality that Flasgot offers out of the box.
The only gripes I have with this extension is that it's not compatible with FF57+ and that it's a little buggy with FF56. So therefore a 4/5 score instead of a perfect score.
To create your own collections, you must have a Mozilla Add-ons account.