They stopped running their own instance that almost nobody used. It’s not the big deal you’re making it out to be. They’re still on the fediverse, they just won’t be maintaining their own instance.
I don’t even see why you think the two things are related.
I don’t really see the big deal. And while it may be bottom of the list for you, I’m sure others might like the tree-planting thing. From what I can gather with a quick search, they actually do it properly, too, not just planting an ecologically dead monoculture of trees.
And yeah there’s an “ulterior motive”, although it’s not really the evil scheming you’re making it sound like. Ecosia paid Mozilla to include them, so now Mozilla has included them as a search option. It’s one of the few ways Mozilla can get revenue, because people sure won’t pay for a browser these days.