As someone in the Apple ecosystem, Firefox is not a very compelling alternative. I've certainly tried: Firefox Focus was my main iOS browser for a few years (I know it's Webkit-based) and I've tried switching on desktop as well. On desktop, various Firefox updates would change my settings subtly and nag me about features I had no interest in. On top of that, it was simply a slower browser, both with DOM manipulation…
There’s platform-agnostic issues too, however. Firefox is my primary browser on Windows and Linux, but it has a pile of UI papercuts which I’ve kinda-fixed with userchrome.css hacks, and the result is underwhelming (as is the possibility of these hacks periodically breaking with updates). It’s enough of a frustration that forking Firefox to properly fix them would be tempting if keeping up with the firehose of security patches from mainline weren’t so daunting.
My biggest wish is for Gecko to re-gain embedding support on desktop platforms so I can build my own browser around it, making keeping it secure as simple as updating dependencies.