So this is basically nothing more than a shell for the Android System Webview (actually Chrome itself in Android N+)? I have a hard time believing this could bring many real-world performance improvements over Chrome, which itself is already a rather light-weight shell. The vast majority of the memory usage is coming from the Webview. Have you done any actual performance measurements? Aside from syncing, what other features does this lack that could give any noticeable improvement?
The most important feature in any browser (aside from it being open source) is the ability to install standard extensions like uBlock, so I'll definitely be sticking with Firefox for now.
I don't know about Android 7+ but I always disable Chrome in phone(s). There are browsers better than Chrome with more features even when they are all using the same Android Webview.
As for extension, there is no such thing like standard extensions. Extensions are browser specific. Ublock for example, it depends on ublock's developers to support Firefox only and not other browsers.