I have been changing the default search engine back to #Google on all installs of #Firefox that I do (and since I push it out via Active Directory at work its more than a few) since #Mozilla made the deal with #Yahoo years ago. It wasn't so much that the quality of the search was poor as it was that more than once the top result of a search on Yahoo was a paid ad serving malware! I just couldn't trust Yahoo. Yeah Firefox had gotten slower compared to Chrome but I don't need to get infected with malware faster anyway.

Yahoo parent sues Mozilla for replacing it with Google as Firefox default search

https://searchengineland.com/yahoo-parent-sues-mozilla-replacing-google-firefox-default-search-287872