I remember seeing that yahoo default option, then having to switch it back to Google (so this was long enough ago I didn't use DDG/Kagi), but I never noticed them remove yahoo outright as a search engine option. But indeed, it is gone.
The official addon (at least its username checks out, but might be a squatted username) appears to be [1].
I can't even comment on why Yahoo! was removed except to speculate that perhaps Mozilla no longer had a contract to give them trademark permission to include Yahoo in their default search engine list?
[1] https://addons.mozilla.org/en-US/firefox/addon/search-and-ne...
