Dark Mode addon - for web pages, not a browser theme -- https://addons.mozilla.org/en-US/firefox/user/11809182/
Nice to have option. Floorp offers a dark mode and tree style tabs under about:preferences#design.
Firefox now has sidebar tabs but not in tree format and LibreWolf doesn’t include that option although it might be able to be turned on. They have a lot of stuff turned off. If there’s a remote concern about security, it’s turned off. There is no prompting to save passwords but you can go in and manually save them.