Pretty buggy, if you have only one computer it works fine, but switching between two computer can sometimes makes them not sync, and create fake/ghost profile. I almost lost a ton of bookmarks. Also the 'not synchronize profile' is also buggued, I wanted to have a profile A on my home computer and profile B on my Work computer, and it started to create a ton of fake profile. the way this add on work is by copying and deleting bookmark from bookmark folder to another, but it should keep the original copy in the switcher folder to not mangled with firefox own bookmark synchro