I'm looking forward to running "real" firefox on ios. If only so that there is another alternative if a page renders poorly in webkit.
I'm not that optimistic that Mozilla is capable of building a good iOS browser. It's been a few months since I last used it, but the issues of Firefox on iPadOS were not because of the engine. Tabs would frequently lose order, closing a tab would close some other tab, broken keyboard shortcuts, cursor/selection issues in the address bar, random non-responsiveness and just janky UI. The issues were so obvious that it felt like it was built without any QA process, so I resorted to using Safari most of the time, which worked perfectly.
Firefox Focus on Android works much better, but it's also a simpler browser. I haven't used the full Firefox on Android, so can't comment on that.
Being able to use Fennec F-Droid was my primary motivator to switch off of iPhone. It's not perfect, but there are things it offers that iOS Safari does not that I consider to be absolute necessities for browsing the web. (uBlock Origin is certainly one of them, also WebM support.)
WebM is supported on iOS Safari now!
It was added to desktop Safari. iOS Safari supports VP9 only in WebRTC. It may have changed, but I can't find any evidence that it has.
If you see it working somewhere, it is almost definitely using the polyfill[1].