Firefox Focus crashes after uninstalling Google WebView
TL;DR: As of Oct 2018, FireFox Focus now uses the GeckoView engine for rendering

TL;DR: Yes FireFox Focus depends on the Android System WebView with plans to switch to GeckoView when its ready. So NO you can't remove the Android System WebView and expect the app to work (as of Jan 2018).

So Firefox Focus is an open source project:

https://github.com/mozilla-mobile/focus-android

Looking at the project, there isn't much code there but does pull a library called: 

geckoview-latest.arr

From what I can tell from this mailing list post (after following a few links from the main page README):

https://mail.mozilla.org/pipermail/firefox-focus-public/2017-October/000018.html

and this issue:

https://github.com/mozilla-mobile/focus-android/issues/13

states that the team started the Firefox Focus project using Android's WebView while waiting for GeckoView to reach Minimum Viable Product (MVP).

https://bugzilla.mozilla.org/show_bug.cgi?id=1322573

The fact that not everything in the Depends On list is crossed off means that GeckoView is a work in progress.