I remember when Firefox used to show Yahoo as a search engine choice in settings. Today it's missing. It appears one can search for one of several addons and hope to get one properly supported by Yahoo. [Edited to be nice.]
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...