In the mobile app - By the same way that you can rearrange open tabs order - Tap and drag. It will be much easier to open the bookmarks you're using more often. I know that lots of people would like to have this opttion.
Exactly! Chrome on mobile allows for this, its a simple hold and drag icon for manually reordering things when they get messy. Why Firefox has decided not to listen to their users on this issue is really a question.