There are about five hundred services that need to do this, to be honest. The separation between a user and an account is one that's largely not valued when people are originally modeling applications, but it probably should be. (Speaking for myself as well here.)
They can run concurrently and are completely isolated. I don't think this is possible with firefox yet. I use app mode since I don't need the normal navigation when using it this way.
and pull up the profile manager (you have to make sure all instances of Firefox are closed first) and you can create as many profiles as you want. Once you have a profile (for example, "worktrello") you can launch it with
firefox -P "worktrello" -no-remote
and it will be a completely separate process, with its own cache and cookies and all.