If you are using old version of Internet Download Manager plugin in New version of Firefox, IDM may or may not work properly. To fix this follow these simple steps…..

Solution:

1] Open your Firefox browser and download idmmzcc.xpi file.

2] Now open this file using your Firefox browser.

Note: If Firefox is not listed in open with box, click Browse button and go to C:\Program files\Mozilla firefox and select Firefox.exe file.

3] Now the Install Add-ons popup dialogue box appears within few seconds and click Ok to install the IDM CC Add-on plugin.

4] After the installation is complete, Restart the Firefox.

source: http://jai-fixyourpc.blogspot.com.au/2012/05/idm-is-not-working-in-firefox.html