I've recently tried a bunch of download managers. Some of them do not integrate with Firefox. How can I integrate them manually? Is there a method common for all download managers?
If the download manager doesn't have support for Firefox integration, then you can install an add-on called FlashGot Mass Downloader. The URL will be forwarded through FlashGot and FlashGot will send it to your installed download manager. FlashGot has a wide support for known download managers.
Read mor on the add-on page: https://addons.mozilla.org/en-us/firefox/addon/flashgot/
Additionally, you can use Internet Download Manager because it supports browser integration for known browsers and besides that, it also features advanced network integration.
Download IDM: http://internet-download-manager.software.informer.com/
Do these addons also collected data about my internet use secretly?
I was shocked at how many recommended programs do this!