Unfortunately Firefox doesn't have a replacement for the "Android System WebView" component, so any app that embeds a browser component (and oh boy is that a lot of them) will still be using Chrome.

There's a relevant ticket here: https://github.com/mozilla/geckoview/issues/167

It should be possible to have a shim that allows Mozilla's "GeckoView" component to implement the API, but - per that ticket, at least - most Android ROMs won't allow alternatives to the Google one.

The Firefox browser is genuinely great, but it's so far from possible to replace Chrome with it everywhere a browser is used.