The default Android browser is no longer supported, and the embedded Chrome is blink-based and less and less webkit-ish.

(And even the different Webkit ports can't be thrown into the same bag.

Compare e.g. Safari on OSX, Safari on Windows, Midori on Windows and Midori on Linux. Two webkit browsers, crass rendering and behavioural differences between them and on each platform.)

It would be better to say that mobile benefits from modern browsers, not just WebKit ones. I've gone without jQuery on a bunch of mobile only projects and run into zero issues.

GP did say "vast majority"

If you want to devote dev time to specialize for the relative handful of folks who run Firefox on their mobile device, that's your call.