Any downstream fork of Firefox. All the good of Firefox and Gecko (including addons), none of the Mozilla corporation. The most popular ones seem to be Waterfox and Floorp (for "most users") and LibreWolf for privacy diehards.

You can copy your Firefox profile folder directly into a fork's profile folder and have everything exactly as you left it (though doing this to Librewolf will likely overwrite some of Librewolf's privacy-first default settings like purging history every time the browser closes)

On iOS you are already stuck with every browser being a Safari+Webkit skin. Even Chrome "Isn't chromium" on iphones. But mobile iOS "Firefox" can still use Mozilla (or self-hosted) sync to desktop Waterfox (etc).

The only alternative is the Konqueror browser (KDE) for the Linux user, it has it's own KHTML engine by KDE (Grandfather of WebKit and Blink)

I am trying Floorp as of yesterday. I like Zen Browser, but their github contributers list makes it look like it's mostly the effort of one person and that always gives me pause until somethings been around a while. Floorp seemed more spread out so I decided to try it despite its silly name.

I'm interested in how ladybird shapes up.

Worth noting that you may have DRM issues on some forks with video content. I don't think you will on Linux, and someone clear this if you can, but I think the alternate used can't do 4k video? I'm not a big web media consumer so idk. Has something to do with Widevine I think.