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Mozilla partners with Ecosia for a better web | The Mozilla Blog
(blog.mozilla.org)
This is a most excellent place for technology news and articles.
I suspect that is something specific to Ecosia, that they changed parameters from time to time which changes the entry, making your selected one no longer available, and then this happens: https://support.mozilla.org/en-US/kb/search-engine-removal
The fact that they announce Ecosia now after it's been available for a bit, makes me think this is what happened. I'll see