Fenix is basically a whole new browser, and that means that all features need to be implemented again (which takes a lot of time, and I don't think Firefox developers have much of it).
Lots of Fennec features are still missing (view source code, save as pdf, install general addons, the whole about:config...). Why they changed is a mystery, maybe the old browser had a core privacy bug or something that couldn't be fixed, but basically they killed Firefox on android.
The web (HTML/CSS/JS/JIT/WebAssembly/Audio/WebGL/WebRTC/etc) engine is exactly the same, the rendering engine (WebRender) is the same, synchronization is the same.
The UI is completely new - that's it.
This is not a wholly new browser, this is a wholly new UI.
I though that fennec was a monolithic implementation for android of the Gecko engine, while fenix has the ui part and the geckoview part separated (so you could create a different app with the same rendering engine, like webview of android).
I have read that geckoview takes the old gecko implementation, probably copied and adapted, but other than that it's a new project with a different development path. That's why I say it's a different browser.
I may be wrong though,I tried to search for sources but unfortunately couldn't find any.
Another missing cool feature from old Firefox for mobile was ability to remove a site url from suggest and history just by long touching on it, which seems to have entirely purged it from the places db.
In fact, the new one doesn't remove things from suggest even if you remove it from your browser history manually, which is also really only convenient to do if you accessed it recently. If you don't want things spamming up suggest, the only option is to use private browsing or wipe your entire history.
