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.
No problem! Tree Style Tabs might also do the job on base Firefox with nested tabs, but it's not as streamlined as Pulse or Edge, especially if you want to hide the tab bar (you have to edit .CSS files).
edit: okay enabling both features, moving the main side panel to the right and enable tab collapsing makes a great space-saving setup.
But that’s also not private as has been claimed as a reason to go FF. The only reason to use FF is only to not use chrome. Not for all the reasons that chrome is bad.
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