Which is why the suggestion won’t work either. Say your computer is infected and you’re not aware. It lies in wait for you to launch Firefox and enter your decryption password, then it takes what it wants. It’s basically a useless idea.
If you have full disk encryption you’re protected if someone takes your disk, but as long as it’s running anything can wait for it to be decrypted. You’d need container/jail/isolation at the OS level to be effective which is outside the scope of Firefox.