A good addon Rated 4 out of 5 stars

Now that Download Statusbar is "sponsored," this is basically your only option for download manager aesthetic tweaks. It's very flexible and adds an info panel, a massive omission in the stock Firefox interface. Yeah, it's a bit retro and amateurish, but not in a bad way (which you will appreciate after Download Statusbar installs a sneaky "sponsorship" system). Hopefully some updates are forthcoming.

This review is for a previous version of the add-on (