I haven't seen this multiple download issue with Firefox, but you may like the DownThemAll! download manager, it provides a queue, maximum downloads, bandwidth limits, and other very nice features: https://addons.mozilla.org/en-US/firefox/addon/downthemall/
Thanks for the suggestion, I already use DownThemAll but it could not even grab the download. It may be the site's fault, once I click on the download link it opens a new tab and the download starts, on chrome I can start another download but it gets stuck on Firefox, it may be that the page was made with chrome in mind idk.
Actually you're right, multiple downloads is working fine now, it might have been V19 or maybe the site was updated anyway its now fixed, thanks for the heads up!