1. History of installing and running arbitrary code on clients in the name of "Experiments" without user permission.
2. Full telemetry on by default.
3. Shoving their products like Pocket (added to the address bar by default, shows up as an advertisement on the page that opens after Firefox updates itself), Mozilla VPN (non-removable advertisement on every Private Window you open) in the name of revenue, while their clueless leadership enjoys fat paychecks for whatever value they bring to the organization.
4. Forcing undesirable changes down people's throat (removed the "Compact" density address bar after a redesign, hiding it under about:config).
Feel free to pile on.
5. Ignoring any and all negative feedback.
6. Taking more and more control away from users.
7. Being funded by their competitor and never really looking for an alternative.
8. Firing a good chunk of their engineers when needed most.
Final straw was the dedicated search bar not working with keyword shortcuts for other search engines. While the settings page claims those are supposed to work in the address bar and the search bar, they only work in the address ("omni") bar.
Too many features removed. I'd rather just go to my native browser and all the inherent advantages to it at that point, which as mentioned is MS Edge. I love it. In fact, their unnecessary additions (MS Shopping) actually opened my eyes to things I didn't know existed, like the Honey extension. I have no complaints about Microsoft Edge on Windows 11 or iOS. After 19 years of Firefox.