Unfortunately, I don't know much about the Scrapbook extension. But if it is like most extensions, then all it's data is located in the Fx profile directory. If that is the case then yes, FEBE would catch it in a full profile backup. The fact that your backup fbu file is so large would indicate it did back it up. (My profile backup with 55 extensions is only 22mb in size.)
One way to be sure would be to restore it to a clean Fx profile and see if it's all is there. Instructions on creating a new profile can be found .