Since many people use their browsers for multiple different things a I do as well, I think it would be a nice addition to be able to choose between different profiles when you start Firefox. This way you could separate different things you do like private normal browsing, business / research browsing, other things of interest that you want to have separated. Also password protection for such profiles would be a great addition. This also allows for age protection. I have for example a Windows machine connected to my living-room TV, when I have guests I would be OK for them to see my YouTube or other things, but I do business research on this machine as well and not everybody needs to see what I am working on.
So profiles on startup of Firefox with optional password protection per profile.
An addition to that could be that you could send pages from 1 profile to another (if you have the password for it)