So I've cancelled the automatic renewal of my Norton subscription because I'm not getting complete value for my money. Where I seem to be losing out is because Norton Internet Security cannot be set-up. This is due to the fact that Norton does not support Firefox, only Microsoft Edge (which I hate!) - see screenshot. Pity, since I've been using Norton for a long time.
I used to separate my System and program files/data on separate drives/partitions for backup and restore reasons. But after getting an disc imaging app I no longer bother with that. I still use a separate drive for data, but I allow all programs to reside on the system drive.
Your supposition about Firefox not being on the system drive could be the explanation of why you do not see it on the list in the Norton interface. But it should not be that big of an issue because that setting is only for what browser Norton will use to open any links from within the Norton app. I have rarely used any links in the 360 app so that setting never really mattered to me.
Your use of Firefox is still protected by all the Norton features. Plus any Norton extensions you choose to use are installed directly in the browser, so those also will function.
So I've run live update multiple times and rebooted the computer and the same problem still exists. I think that part of the problem is that Norton thinks EVERYONE has a large enough C: drive to hold their system files as well as all their other data, since when you install Norton it automatically goes onto C:. I have a very small C: drive (designed for the system only) and a 1Tb D drive, so when I downloaded Firefox I put it on the D drive to save space for system files. So with Norton installed on C:, I think it only recognises MS Edge as being the only browser present because Edge is also installed there. It would be really nice if Norton would get with the times and start giving their clients a choice as to where they want their program to be installed instead of automatically assuming that it always has to go on a C: drive! I just checked my laptop (which only has a C: drive) and Norton Internet Security is happily running on Firefox! Talk about frustrating!
I have no issues having Firefox as a selection for the Norton interface and the default browser that will open from links from within the Norton interface.
Have you ensured that your Firefox is up to date?
Have you run LiveUpdate manually a few times, restarting as necessary, until no updates are available? Then restart again. Restarting, not shutdown and startup again if you have Windows Fast Startup feature enabled. If you use Windows 8/8.1 or 10 or 11, there can be an issue with the Windows Fast Startup feature.. See more information here.