Is there any Internet Download Manager (IDM) extension works on firefox 6 beta?

DownThemAll works for those versions.

FlashGot is the free add-on for Firefox and Thunderbird, meant to handle single and massive ("all" and "selection") downloads with several external Download Managers.

https://addons.mozilla.org/en-US/firefox/addon/flashgot/
http://flashgot.net/

Compatible up thru 7.0a1.

There is a working version that comes with IDM 6.07 build 8, IDM CC 7.3.5

1. http://www.mediafire.com/?d8su11xtu8otxgu or

2. http://www.internetdownloadmanager.com/idmmzcc/idmmzcc.xpi

the second one can be found in this page: http://www.internetdownloadmanager.com/support/firefox_integration.html

This works for the Firefox 6 final as-well.