Stop Firefox from taking over downloads
Some websites allow only one download request. I use IDM {Internet Download Manager} to down load files. So if Firefox makes the first request IDM will download the web page instead of actual intended file. There is an option in IDM to force downloads but this doesn't work for the above request. Is there something I can change in [about:config] to stop Firefox from Downloading when I click a download link then IDM will have total download privilege
Opaite Mbohovái (1)
No, I can't think of any setting that when you click certain links Firefox should go ahead and load that link, and in other cases it should not load that link. As far as I know, a link is a link until the website responds and indicates what content type it is. By then, I think from your description, it's too late.
If you just let Firefox take that download, does it work? Or does IDM cancel it and re-request it and that's the problem?
Can you get support for the authors of IDM on what they suggest and let us know what you find out: https://www.internetdownloadmanager.com/support/index.html