yea, that's a pretty good reason :) afaik the list of what containers separate is here, if you're interested: https://wiki.mozilla.org/Security/Contextual_Identity_Projec... (I can't speak to its up-to-date-ness tho, I'm not familiar with how well maintained the moz wiki is)

there are definitely some leaks, and there is a "facebook container" extension which plugs some (all?) of those... but yea. if you want maximum separation, you want maximum separation, profiles are probably always going to be the safest bet.