It turns out there are two CDN's that can provide this: snowpack/skypack, and jspm:
skypack:
import firebase from 'https://cdn.skypack.dev/@firebase/app'
import 'https://cdn.skypack.dev/@firebase/database'
jspm:
import { firebase } from 'https://jspm.dev/@firebase/app'
import 'https://jspm.dev/@firebase/database'
These both deal with "bare import" conversion to JS, and any code conversions required to be JS Modules.
Google does not appear to want to support a module form on their firebase CDN, an Issue to that effect was immediately closed, suggesting complex workflow solutions.
I'm really thankful to the two projects above, supporting simple es6 JS and zero workflow solutions.
Edit: snowpack's esinstall can turn most js files into modules. Here's a node script that brought in all my dependencies:
#!/usr/bin/env node --require esm
import { install } from 'esinstall'
import nodePolyfills from 'rollup-plugin-node-polyfills'
async function run() {
const foo = await install(
[
'mapbox-gl',
'three',
'three/examples/jsm/controls/OrbitControls.js',
'three/src/core/Object3D.js',
'chart.js',
'dat.gui',
'fflate',
'@turf/turf',
'stats.js',
'@firebase/app',
'@firebase/database',
],
{
rollup: {
plugins: [nodePolyfills()],
},
}
)
}
run()