Thank you for developing this, Download Status Bar was a great add-on in the old XUL/XPCOM standard that I used in old Firefox, and it's nice that we finally have a functional status bar in the new WebExtensions format.

Here are some of the things that need to be added (some have already been mentioned here):
-Download speed bandwidth, this was available in the old add-on
-Reduce bar height to how it was in the old add-on, it's way too big right now
-Polish the UI a bit more, it doesn't look like a particularly finished product
-Possible bug, the completion sound doesn't play

I hope to see this add-on grow and improve. Good luck on this project!

Firefox 57.0.2 x64