It is not true at all. After a long time Microsoft's browser share in increasing. They probably are quite happy about it,.
Having more instrumentation over the platform they're doing it on is probably reasonable and realistically blink is already on every platform with over 1% usage share so that's a much easier road than supporting Trident, EdgeHTML or Chakra - the engine is not their core product - it's the stuff built on top of it.
I've used edge on linux - other than the weird thrill of seeing software that says "Copyright Microsoft" running on linux it's just a typical functional webbrowser -- that's exactly what they need.
I read a post here on HN a few months (years?) ago where a developer was really trying to use Gecko for their own application and got frustrated so much that they gave up and switched to Chromium.
It predates KHTML/Webkit by several years so some additional legacy baggage is to be expected.
I see the main asset of firefox as being
a) not google tech
b) mobile browser with ublock
c) tech nerds with tons of custom FF addons, never wanting to switch
So I don't see them go away anytime soon, but I also see nothing to stop the decrease of their marketshare. That means, unless mozilla would focus on their core values again. They lost much trust with quite some shady actions now, over the years.
Imagine what kind of HN thread pops up if Firefox is abandoned by Mozilla.
Also Firefox is a single licensed product that is MPL, and for anyone who wants it that way commercially may support it.
Microsoft basing Edge on Chromium was a really stupid move on their part for this reason, and I suspect they are starting to realize it.
That being said, I only expect it to be maintained at a hedge level, not enough to shoot for dominance again. Think of it like Valve's Steam on Linux efforts -- it's not a major effort on their part, it's just enough to keep Microsoft honest.