Primary Password is unlock after reinstall Firefox. I can see all of my password in there without trying to do anything. And it can do the same thing when you import from another browser as well.
This sounds like a bug, reinstalling Firefox should not remove Primary Password.
Just to confirm, when you reinstalled Firefox, did you sign in to Firefox Account? Primary Password is a "device password" or "local password", it's not synced. On new profiles there is no Primary Password and signing in to Firefox Account will bring passwords to that profile. Perhaps this is what caused the confusion?
Feel free to file a bug here https://bugzilla.mozilla.org/enter_bug.cgi?product=Toolkit&component=Password+Manager and we can investigate!
To be sure, I reinstalled it again. And it seems like the Primary Password (Master Password) and all of my data remain the same. So I tried to reinstall it once, but this time I also removed data in "%appdata%" as installing it on a new device. Then it turned out that all my data was removed from the browser. So after that, I logged in to my account, and it bought the "Saved Logins." back. But not include Primary Password. I'm not sure. Is it a bug? But If it isn't, I think it should warn people to set it or use a firefox account to see detail.