I feel like having the ability to open Firefox on any device (that you are signed into), write down notes and/or set any reminders you need, then have Firefox notify you about them either after a specified time or whenever you are on a specified device would be a great feature.
Myself and many other people I know always remember things at times where they can't do anything about it then forget the moment they get to where they need to be. Having the ability to have Firefox remind me things would be extremely useful because people always have their browsers open and it seems like the best method of having a reminder sent to yourself.
It would also be great if you could use your phone (or any other supported mobile device) to take notes/reminders while offline and have it register the moment it is able to access the internet.