The best replacement for "Download Manager (S3)" that I've found so far, first of all I'd like to thank the developer for his/her hard work doing this for free for all of us to enjoy! :)
It does need some polishing, so if you could consider my suggestions I would appreciate it:
1) As many have said, the bar is too thick right now. It gets even thicker when downloading many files at once. Also, it changes size when you zoom in our out in a tab. And when you change to another tab, the bar either disappears or changes size according to the zoom level of each tab.
Please fix this making the bar size smaller, not dependent on each tab's size, and visible in all tabs/browser.
2) I'd also like if you could add some more info, like file icons, download speed, percentage... And that you could choose what info to show.
3) Add an option to "retry" when a download fails.
4) Add auto-clear option.
5) Add option to change color. Maybe even to the Firefox theme you are using.
6) Sometimes a download does not appear on the download bar for some reason (but it does appear on Firefox download history). Looks like a bug.