How is Mozilla choosing which search engines to show in their browser oppressing of free speech?
"Freedom of speech is a principle that supports the freedom of an individual or a community to articulate their opinions and ideas without fear of retaliation, censorship, or legal sanction" https://en.wikipedia.org/wiki/Freedom_of_speech
Are they blocking users from going to those sites? No. Are they preventing those sites from loading in the browser? No. Are they stopping anyone from creating a website and saying whatever they want? No.
Just because you don't agree with Mozilla on this action doesn't make it oppression of free speech. This is simply one organization deciding not to have a relationship with another organization. Not everything is an attack on free speech or expression.