I experimented with this profiles feature a while ago but since I realised syncing multiple profiles resulted in the bookmarks homogenising across all profiles (exactly what I didn't want) I went back to just one profile.

First, when you have multiple profiles you can't delete the default profile. When you create a second profile, Firefox automatically makes that the default profile. This would be why you can't delete that profile you now don't want. Go to your other profile as in your screenshot and edit it to make it the default. Then you can delete the other profile and just have one profile.

Editing doesn't work either. Sync is a no go option for me since it once messed everyting on my browser.