how to stop Yahoo from answering questions in Firefox?

The simple way is to set a different search provider: Add or remove a search engine in Firefox

If it keeps coming back it might be more convoluted (as that can mean some malware either on system level, or a dubious addon that keeps re–setting your searches for their monetization purposes).

What do you mean by Yahoo answering questions in Firefox?

Are you saying you are doing a unnamed search engine search in Firefox and Yahoo takes over?

No version of desktop Firefox has come with a Yahoo search engine since Firefox 67.0 so it had to have been installed by an extension you have installed or updated recently or by some external software or malware perhaps.