Another option is setting Firefox to open the profile manager[0].

That's what I've always done when I don't want to use multiple accounts on the machine itself.

[0]https://support.mozilla.org/en-US/kb/profile-manager-create-...

Have you considered just creating multiple windows/mac accounts? Since you can switch without having to log off, it's really convenient. I understand partners sharing accounts and such but it's really just a better workflow/experience IMHO.