If I want to opt out, I can only opt out by disabling all usage telemetry according to the official doc https://support.mozilla.org/en-US/kb/share-data-mozilla-help-improve-firefox . That means that my usage habits and the usage habit of all people that do not want to share their search terms with firefox, i.e. all somewhat privacy interested people is just thrown into the void. For future browser decisions, this means that the browsing behavior of a whole group is neglected. This makes me sad.
That wasn’t the point of OP’s post. They’re saying that people who want to opt out from this search data telemetry must also opt out of all telemetry, which means privacy conscious users will be under-represented in Mozilla’s telemetry data (selection bias). This could lead to worse privacy-related decisions from Mozilla in future.