I was quite long searching for a good GUI download manager for ubuntu. I used Flareget, uget etc. but then I came upon an add-on for firefox named as Downthemall. You can find it here. It is best but I have one problem. When power cuts off, computer powers off. When I restart computer and restart mozilla firefox, I am unable to find the downthemall download manager window. I start another download and thus window pops up. Any way to make the download manager window automatically visible when download is in progress. Thanks for support.
Try clicking on the Open Menu button in the top bar:
Then, click on the Customize button:
You should see the DownThemAll Download Manager somewhere there. Simply drag and drop the icon to your task bar or into the Menu! Then, you can just click that button and you're good to go!
I don't use DownThemAll myself, so I don't have it installed and can't be entirely certain that will fix your issue, but it should.
If found a command that works for Linux and OSX, just run on terminal:
firefox -no-remote -chrome chrome://dta/content/dta/manager.xul
Then you can create a shortcut and customize the icon, etc.