The first 'Send to Download Manager' addon I've come across that doesn't have to do a bunch of weird bullcrap to work. You just need the Download Manager and this addon, nothing else.

My only criticism is that the 'Interrupt Downloads' feature isn't very helpful - it's weird to choose where to save the file, only to have it caught by the download manager and it's going to put it in a completely different place.

But the context menu option doesn't rely on Interrupt Downloads and it works just fine.

The Download Manager itself is based on aria2, so the download speeds are very fast. This is definitely the best solution for Firefox Quantum right now.