Feels like we need something like early versions of Edge, where it was using Chromium but could be told to open individual tabs (or configured to always open links to certain origins) in an IE webview.

Except, instead of Chromium, Firefox, and instead of IE, Chromium.

I assume you mean early edge2. The first release of edge (project spartan) used a custom rendering engine and javascript runtime. The chrome-based version released five years later.

It might have died with e10s but there was a firefox extension that let you embed IE in a tab on demand or for certain sites.