Firefox just adds incompatibility to the mix of things you have to support, frankly I'd switch to Firefox if they decided to build it on top of Chromium.
When they were actively working on Servo and had devtools team I could see the potential, but after they sacked those - what's the point ? Market share is shrinking so compatibility is going to get worse, devtools are worse, performance/stability is worse in my experience.
But at least Mozilla do a basic back-of-the-envelope “is this feature a huge security vulnerability” check before shipping. (Looking at you, <portal>.)
It's hard to get behind engine diversity when Mozilla is trying to take the web in a direction I don't really want.
Meanwhile Brave does the same thing, but with Cryptocurrency.
I wish we had an open source clone of Vivaldi.
FLoC seems to be pretty irrelevant to anyone not directly working on ad infrastructure, so I'm fine with Mozilla ditching that.
The value of IE to Microsoft was to lock people into Windows - both developers and users. They recognized that the web would become a software platform in its own right, and knew that if everyone ended up using web software then it'd be a lot easier to use non-Windows platforms.
Google gets much of its revenue from web advertising, so the value of Chromium to Google is to keep the web profitable for them. For example, Mozilla can't do to Google what Apple did to Facebook. They can do things like veto any move away from cookies as the primary tracking mechanism, at least until they have an alternative ready to go.
An open source license doesn't fully solve the problem because forking Chromium isn't enough. All the software on the web is built on top of APIs that Google gets a say in, and these are designed to be useful for ad/tracking networks.
WebKit only matters because of iOS, and Blink and WebKit still share a lot of DNA. It's not a credible competitor otherwise.
I do agree killing Servo was a shortsighted move, but, speaking as a long-time Mozilla community member, that's hardly the only shortsighted move MoCo has ever made (see also: dumping embedding for Fx4).
They also wanted to get superior compatibility. While the old edge was quite a bit better than IE, it still feel short.