I use Norton Security (their current version, after upgrading from Norton 360) and find that as long as it isn't set to Silent Mode, I get prompted for all firewall and other issues after upgrade or updates, or Norton automatically sets the rules correctly, but sometimes it takes a couple of attempts to catch all issues. As Jim mentions, Defender still runs at PC Boot time, even when Norton is the selected Firewall & Antivirus. Microsoft seem to do that now to prevent bad players getting a toe into your PC before a third-party security solution can launch.
This is what I would do if I was in you current situation:
1. Backup your library.
2. Uninstall both MC23 and MC24.
3. Delete the MC23 and MC24 program directories.
4. Reboot.
5. Reinstall Norton Security, check settings to make sure it isn't in Silent Mode by default. The remaining defaults are probably fine, but up to you.
6. Reboot.
7. Run Windows Update.
8. Reboot. Let any updates complete.
9. Run Windows Update again.
10. Reboot. Let any updates complete again.
Check there are still no MC program directories. i.e. Nothing has been restored as per point 13 below.11. Install MC24. Respond appropriately to any prompts from Norton Security.
12. Reboot.
13. Check MC24 starts fine. If not, something really weird is going on, like Windows is restoring protected files after they are deleted during an uninstall. Or perhaps if you have a Backup solution running, it has similar functionality and is restoring deleted components silently.
14. If point 13 was all fine, restore your library including settings, stop and restart MC24 to make sure all is good. (Optional reboot here.) Play music, video, TV, Panel, Gizmo, etc. if you use them, and respond to all Norton Security prompts as appropriate.
I suggest the above because I have seen Windows restore out of date protected files, and I have seen Windows forget which version of MC components it should be using through file associations. Not to forget Windows Defender is often a mystery in itself!
Oh, while doing the above there maybe be a step;
3a. Delete the "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 24\Plugins" directory, particularly if you use any non-standard plug-ins. It wouldn't hurt to do it anyway, as MC will redownload LAV, madVR, etc. as required. You would just need to be sure you had sources for any non-standard plug-ins.
Good luck.