Well, as it says in the article, Firefox and Chrome on iOS are both just WebKit wrappers, because Apple doesn't allow other browser engines into their store, so I don't know what kind of black magic or bias or whatnot your users are experiencing.
As for Android, the new CSS engine (called "Quantum CSS" or "Stylo"), which landed with Firefox 57 on the desktop, should ship on Android with Firefox 59.
There's also "Quantum Render" or "WebRender" upcoming somewhen soonish. I'm guessing Firefox 60 or 61 on the desktop, so maybe again with a small delay then on Android, I don't know.
WebRender should be kind of a big deal. In extreme cases, the difference can look like this: https://www.youtube.com/watch?v=u0hYIRQRiws