why is Firefox using Yahoo as defult browswe?

Firefox is a web browser while the Yahoo you are seeing in Firefox may be as a Homepage or as a search engine? https://support.mozilla.org/en-US/kb/how-to-set-the-home-page https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox The desktop Firefox web browser has not come with a Yahoo search engine as one of the default options since Firefox 57.0 and later versions. So if this is Yahoo set as a search engine then perhaps an extension you install ed or updated recently in Firefox added Yahoo.

That's probably due to some unwanted extension or malware, please see support.mozilla.org/questions/1590908 for possible options.