This is getting off the OP subject but to those interested I not had any problems with any of the FF betas. If you want to take the plunge the only hitch may be an Extention not working. The widely Extentions are keeping up with the beta releases.

I will let Chris speak for the beta beta but I am guessing they are just as stable as just the betas final releases.  These releases seem to be more stable then Final releases were in the past.