Am using ff Quantum 60.0.2 (64-bit).
Ok; FDM actually works somewhat well. The installation is STUPID.
I found this download manager from ff, but with some difficulty, perhaps other extensions paid ff more?
As usual, you have to add this extension in Firefox, (later, after I downloaded the real program from its website, and investigated the rather limited FDM settings, I found that FDM didn't know (button indicated no ff installation) that it had been added to ff, and asked me to check to see. I did and it led me to ff, where I saw that it was added to ff.
You will see a little blue icon on your FF toolbar.
I was not able to do anything with it.
Then I went into FF extension listing and doubleclicked the name which took me to the FDM website where I was presented with a download for FDM.
After downloading the FDM for my computer, I ran the FDM downloaded installer file (Norton approved it).
By the way, an icon will appear on your bottom screen bar, and when you click it you will be asked if you want to add it to Chrome. I said yes, and thought for a period of time that it had lockedup the computer because there was only a partially formed window and no progress bar. But it finally completed, and I closed Chrome.
Then I was left with a FDM window with a big box saying Drag and Drop. You can forget that.
Now the FDM actually works, but again, stupidly.
I went to Youtube and looked at a couple of kirghiz folk song videos.
Now adays, almost no videos provide a download function.
I tried various drag and drop, and cut and paste Url techiniques but none worked.
Finally, I clicked opened the upper left horizontal bar stack on the FDM window and found paste urls from the clipboard. I had previously loaded the clipboard with one of the video's url.
Then I could get the video to download rather quickly (and successfully. later I used real-audio to extract the Mp3s), I also pasted several other similar urls but only two downloaded, the other two gave me err msgs "file not found". Perhaps I had closed the FF tab for the youtube or perhaps FDM failed to latch onto those videos.
Note that there is a (default) option to skip confirmation of downloads (useful, I think).
The other defaults appeared to my eye to be reasonable.

The FDM window has a + tag which permits only one (I think) "tag"/per + action. This maybe for mid mp3 pdf extensions but I haven't had a chance to check it.
By the way, Where Is The Instruction Manual???

By the way FDM email help desk is apparently still functioning and rather rapidly. Get help before it gets too big or too good.