./mach npm install --prefix=browser/components/newtab, which comes straight from https://firefox-source-docs.mozilla.org/browser/components/newtab/docs/index.html has various problems on Windows 10. Among others it doesn't want to install the dev-dependencies.

The simplest workaround is to, instead, do this:

(cd browser/components/newtab && ../../../mach npm install)

Why this is happening is unclear. Probably the simplest thing to do is to update the docs.

This may or may not be related to the Node 12 upgrade; I'm not sure.