Unfortunately Firefox doesn't have a very good Profile Manager, the onboard one is very primitive (unlike other browsers).
I tried this instead, and it works well. I have different subject areas arranged in different Tab Groups. It is easy to stay organised, and switch between subject areas.
Works really well for keeping your Tabs organised.
Has a built-in backup tool which I regulalrly use!
Update: I now use shortcuts to open the 'Groups Manager' for ease of managing, also for a manual backup, before I make any changes.
Great tool for organising different subject areas while you're working.