I didn't see anything about IE being dropped.

I certainly hope not. There are still several business apps that need to use IE.

Cisco wireless controller management for one...

Firefox is still using Gecko, their in-house rendering engine since the beginning. The reason MS supposedly shifted to Blink (we don't know yet) is that they want to fill MS Store with PWAs (Progressive Web Apps) and the reference PWA platform is Blink/Chromium/Chrome. At least that's what I think.

Since Chromium is open source and the place where development happens MS could fork it at any time and apply their own tweaks, which they probably will do.

I think this is closely related to PWAs and if it was just about the browser it wouldn't have happened. Edge wasn't successful so it kind of makes sense to not spend a lot of money on it. So this is actually one of the few times that I think Microsoft is doing the right thing.

Nothing will happen to IE. It's a legacy browser at this point and will be kept the way it is for legacy compatibility.

A big shame, in my opinion. I'm not particularly a fan of Chromium-based browsers. I mainly use Firefox, but I really like Edge a lot and it's my 'second' when I need a browser that isn't Firefox to use. I'd probably use it exclusively if it had support for NoScript.

Brave's muon-based browser had been my fave, but they've forked to a chromium-based browser, too.  At least their flawless page rendering has been maintained in the chromium version.  PDF handling works correctly on all my work-related sites with the chromium version, unlike the muon version, & the few Chrome extensions I liked are usable now.  So at the moment I've moved over to Brave's chromium version as my default.  Everyone's gonna be selling ice cream, just different flavors.

I've had it with Chromium based browsers. I used Opera for a bit until it started acting like Chrome, taking its time to open, going black before fully rendering and only on one particular site. Tried Vivaldi and that was a total bust. It took longer than Chrome or even Opera to open, minutes sometimes and it too is Chromium based. Now I've gone back to an old friend, Firefox. Its stable, opens quick and clean and doesn't screw things up. Edge won't work worth a shit. Opens up very fast and closes just as fast. Won't stay open long enough to do anything. On the other end IE actually works and its even skinned. So that's it. No more playing with that other garbage. I got things the way I want them and that's my story and I'm sticking to it. 

Official confirmation:

https://blogs.windows.com/windowsexperience/2018/12/06/microsoft-edge-making-the-web-better-through-more-open-source-collaboration/

Mozilla isn't very happy abput this development:

https://www.theverge.com/2018/12/6/18129287/google-microsoft-edge-chromium-response