IDM CC Firefox Addon for Firefox 7 and 8.
Here it is.
Extract the .zip file, go to browser's add-on management page, click on the gear icon and go to the option to install add-on from file. Then, choose the XPI file and you'll be fine with Firefox 7 and Firefox 8 and IDM.
Download IDM CC Firefox 7 Add-on
Update: This add-on works on Firefox 8 as well.