I stick with zen because of spaces. I have a few, e.g. common browsing, work or serious topics, finances, shopping. Each of them I assigned default containers that they will run. And each if them have different pinned tabs related to the spaces.

It's been so long since I switched that I honestly forgot how Firefox base is. I think before I switched I used sideberry to do all that spaces thing but zen is more integrated feeling.

There's just something that hits it for me that it's indispensable for me.