In the recent Firefox-es, some java scripts are disabled. The download manager seem to use a java script that is not supported by the new releases.
Can you please check if there is any way of using Firefox?
All I can tell you is to take a look at the Download help page (https://account.mentor.com/help/download). If you need further assistance, please open a help request by clicking the green “Request Website Help” button in the upper right. This will ensure that the correct people are able to respond to browser/download issues.