As per the documentation given in Internet Download Manager's site, you do not need to press any special keys to use it with Firefox (see here: http://www.internetdownloadmanager.com/features.html).

Check this FAQ for more information on integration with Firefox: http://www.internetdownloadmanager.com/register/new_faq/bi_main.cgi?br=Firefox

For further clarifications, you should contact them: http://www.internetdownloadmanager.com/register/new_faq/main.cgi

Sorry, but I am compel to response to you blatant, that you din't get my point: "How to configure Firefox with Internet Download Manager and/or to set Internet Download Manager be automatically be the one to download instead of the browser built-in downloader?" Actually I've been there long before on all sites you are mentioning and they informed me that they are working for it and advice to use the special keys to preven and/or force to download and/or even by drag and drop in order to download using IDM. That's why I am trying to figure out how if there's any to disable the built-in downloader of the Firefox 6.0 in particular so that when just simlpy clicking in on the object to be downloaded it will work like the older Firefox and its counterpart the newest IE9 without any hassle of configuring IDM. Thanks anyway.

OK, now I understand. If you had mentioned that you are already in contact with the IDM support, it would have helped me understand you earlier. Anyway, the browser integration has to be done from IDM side, so I would suggest that you continue to talk to their support.