I worked on this project - Web Components had a lot to do with it. It's true they weren't feature complete in Firefox when we started the project (notably missi

I worked on this project - Web Components had a lot to do with it. It's true they weren't feature complete in Firefox when we started the project (notably missing Shadow DOM) - but the first migration from late 2017 was XBL->Custom Element (https://bugzilla.mozilla.org/show_bug.cgi?id=1500626 https://bugzilla.mozilla.org/show_bug.cgi?id=1500626), and we used the migration project to inform performance and feature work for Web Components within Firefox. We could have converted the bindings to JS modules (and we did plenty of that too), but Web Components were key because they allowed us to migrate in-place with near API compatibility rather than rewriting the UI. I have a post at https://briangrinstead.com/blog/firefox-webcomponents/ https://briangrinstead.com/blog/firefox-webcomponents/ with more detail on that point.