Putting Facebook in a box is easier for many people because they have fewer things that are connected to their Facebook. Any such things break with containerisation. Firefox spells that out, but people will be surprised anyway.
Let's give a real world example. I use Facebook container. I also have YouTube premium. So, normally I don't see YouTube adverts because I'm logged into my account with Premium.
But inside the Facebook container, I am an anonymous YouTube user, with no Premium account. So if a user embeds a YouTube video, it has adverts.
Now, if I used the Google Container, all YouTube videos, other than on YouTube itself, would be anonymous and so have adverts.
There's a lot more integration like this for Google than Facebook, and so the experience with containers is worse for Google than Facebook.