I'm curious, too. The blog post says refreshing the page won't revalidate the immutable resources, but doesn't say what happens for a CTRL+SHIFT+R hard refresh.
The Firefox bug mention hard refresh, but don't say what was implemented:
Patrick McManus, the Firefox developer of this feature, confirmed that hard reload will load immutable resources from scratch, so the user always has a nuclear option for fixing cache corruption. :)
The Firefox bug mention hard refresh, but don't say what was implemented:
https://bugzilla.mozilla.org/show_bug.cgi?id=1267474