Type of Browser won't have any relevance on downloading in manage content as it's not used, so looking at something common to both download methods, doesn't appear that Comcast are blocking FTP according to their block list.
So could be the Firewall. CM or Manage Content needs to access the DLS initially using FTP on port 21, check Firewall and see if 21 is blocked.
This is from checking what TRS19 is accessing when downloading I used CurPorts from Nirsoft.
Code:
TRS19.exe 4812 TCP 2513 192.168.1.65:2513 [COLOR=#ff0000][B] 21 ftp[/B][/COLOR] 107.182.226.63:21 firstclass.auran.com:21 Established F:\Trainz Railroad Simulator 2019\TRS19.exe 0.0.0.0 N3V Games Pty Ltd 11/02/2021 13:16:19 PC01\Malc A 11/02/2021 14:21:53 Trainz Railroad Simulator 2019 Launcher
Or you could try
Easy way if using the Windows Firewall is to reset it, then when you launch TRS19 or any version of Trainz you would get a popup asking if you want to allow it internet access or not.
Incidentally for Firefox users the FTP button does work in Firefox 85.02 here, as FTP is manually enabled. see about:config then search network.ftp and enable it. This will work until Mozilla completely remove FTP.
Microsoft Edge had blocked FTP for some time, I believe that if you use the IE mode it may still work, don't use Edge so no knowledge on how.
You can however still access the file using an FTP client such as Filezilla, using the info from the download page, somewhat long winded and you need to know what you are doing!
FTP has had a security bug for at least a decade which is why it's now getting killed off, SFTP would have solved the problem however seems no one is interested.