The real death knell is that Microsoft decided not to go with Mozilla in building the relaunched version of Edge.

That would have been a very fruitful relationship, but they couldn't make it work. My understanding is - albeit its second hand - that they really didn't want to simply jump to Chromium, but Firefox proved far more complicated to do what they wanted to do.

Ultimately, Microsoft Edge went from a pretty good browser to loaded with of things I dislike, which is a real shame, but I know it would have significantly boosted usage numbers of Firefox and its engine, which in turn would drive more investment into Firefox itself.