Tor Browser, LibreWolf, and Arkenfox JS are the most secure and private browsers you can get and they're all based on Firefox. If they're not private enough for you, I don't know what is

I think the management's recent decisions, as well as the removal/lack of power-user features for those users, have moved a lot of people away from Firefox, myself included. They really need to focus on providing really good software, not get caught up in trying to chase trends or forcing services people don't want. This WIRED article does a good job explaining the issues.

I am keeping an eye on Pulse Browser, which is an experimental fork of Firefox with uBlock Origin pre-installed and some UI customisations. They've got a sidebar with "web panels" very much like Vivaldi's Panels, and they've got vertical tabs like Edge. People also seem to be posting suggestions to their discussion page on GitHub. It's early days, but if they listen and try to implement some of the suggested features to their best ability, it could be a much better Firefox than Firefox itself.

Edit: From Pulse's Discord, I just found out Mozilla bought Fakespot and are integrating that into Firefox. Some people are definitely going to think that's bloat.