Hi, Interesting. That could be that IDM https://getidmcc.com/ has not yet had a update to be compatible with this version yet. Suggest go to Add-ons --> and look to top of page right corner for the Gear Icon and Click and Check for Updates. You will have to check that again or it will update if not please go to their forum to find out as Mozilla only ok's Extensions submitted by Developers to be used with Firefox and give the Developer a web page to distribute their Extension from. This in no means that Firefox Support does the Developers Support when their page has a Support link and a contact Link. Please let us know if this solved your issue or if need further assistance.