You can add Ecosia as a search engine to Firefox Android by going to Settings - Default Search Engine - Add then entering the following:

• Name - Ecosia
• Search URL - https://ecosia.org/search?q=%s
• Complete URL - https://ac.ecosia.org/autocomplete?q=%s&type=list

This will also allow you to use the Firefox Search widget on your home screen to search Ecosia.

I tried using the Addon to no avail, so I had to manually add the search myself.

Thanks for creating the community! I had forgotten to use Ecosia on Android for a while now. So much missed opportunity