Why? Battery. Efficiency. On the new M1 Macbooks I can run Safari and be unplugged all day long. The increased power consumption and battery drain is noticeable when using FF and I can't make it all the way through the day. As far as extensions go, I have 1Blocker and it works well, it does block YouTube ads. The only things I miss are Tree Style Tabs and Container Tabs.
I also find performance on FF to be a lot worse than both Chrome and Safari. Firefox gets sluggish with many tabs open. JS-heavy sites will be jittery and laggy in Firefox, but buttery smooth in Chrome. Having 100 tabs open is sluggish in Firefox, but smooth in Chrome/Safari. In FF some sites will occasionally crash their tab, but they don't crash in Chrome/Safari. Etc.
Then there's the attitude of Mozilla constantly changing the interface, shoving things like Pocket or Mozilla VPN in my face. Sorry Firefox, but I've left and not coming back.