Not only Mozilla's privacy claims are smoke and mirrors (you need uBlock Origin to get any reasonable privacy on the web today - why is it not included by default despite being permissively licensed?), they have opt-out telemetry (in breach of the GDPR), the browser will load a page with Google Analytics on first run and will nag you with bullshit on nearly every update (and it updates often) not to mention change the UI all the time for no good reason.

Firefox is the most user-hostile free & open-source software that I know of, and more hostile than a lot of paid, proprietary software.