You might not agree with this but hear me out. I used to work on Chrome and I was curious what the justification was for storing passwords unencrypted in Chrome. I went and talked to the Chrome security team and they frustratingly explained it to me as they get this question all the time.
As you said, in FF, (and in Chrome), a few clicks and anyone could look and an unattended computer and see all the stored passwords. As they pointed out, an unlocked unintended computer can pretty much be owned by anyone who wants to own it. You've given them access to your computer. They can open a shell and start running apps. They can exploit any bug in the OS or other app. They can copy files to a USB stick or across the net. On top of that, your password manager (or FF or Chrome) will let them log into your mail, your bank, your facebook, whatever services you've saved passwords for even without knowing your password.
The point is
(1) don't leave your computer unlocked and unattended. Put a password on it, when you walk away from the computer lock the computer (start the screensaver or whatever that makes it required you to use a password to get back in)
(2) don't ever let someone use your computer logged in as you. If you hand someone your computer to use login as guest then hand it to them.
If you're like me you'll probably reject these suggestions. I thought "I don't want to be bother to lock and unlock my computer all the time" and I thought "It's stupid to expect me to put my computer in guest mode anytime I let someone else use it."
But, after I calmed down and thought about it I realized they are right. If someone wants your passwords or other data and you hand them an unlocked machine they are going to get them. How FF or Chrome or Password managers store passwords has nothing to do with that.