I use Firefox but I disagree with them that MS switching to chrome is a bad thing.
It eliminates from the equation a browser implementation that never really was that popular and at best sort of did what other browsers did without actually doing much different.
You have to ask what the point for MS would be to continue to fund that. It wasn't buying them much positive differentiation. They had a little, mainly in the form of some performance benefits. But mostly people ended up using Chrome or Firefox, even on Windows 10. Mostly it was differentiating negatively for them in the sense that it had its own sets of unintentional compatibility bugs that most web developers did not prioritize working around.
Several years into the windows 10, it's been largely a success for MS. Edge is the exception however. They made a few strategic mistakes with it (still under Balmer), the primary one being to make it windows 10 only. This made some sense when the plan still was windows 10 on every device but that went out of the window when Satya Nadella unceremoniously killed off windows phone. So while windows did well, Edge did not. Worse, MS now had to support pretty much everything they did on Android, IOS, and Mac OS as well. So, having their own browser engine just made that more complicated for them without offering any benefit. Porting it to other platforms would have been a serious investment, without any clear benefits or a path to success.
Arguably, already at the time you could have questioned the logic of putting vast resources into duplicating the efforts of several open source browser engines with the explicit goal to do exactly the same functionally. IMHO it was largely the not invented here syndrome that drove Balmer/MS to do this. The new found pragmatism under Nadella produces different decisions. So, they keep the UI but they swap out the internals for of the shelf OSS that works perfectly fine. Happy users, happy developers, less cost. Not the hardest decision he's had to take I imagine.
As to Firefox, they have an ongoing project to re-implement their engine in Rust that is producing clear benefits in addressing pain points in their old implementation that are also common to other C++ based implementations. They've been doing a great job staying on top of web standards and are a good contributor to them, often pioneering features such as wasm or webauthn in Firefox first. So, their implementation and focus on privacy adds value and they have lots of users that appreciate what they do and how they do it.
I get their frustration that Google is gaining more power here where MS used to provide some independent voice in e.g. standards committees. But lets be fair here, MS wasn't doing a whole lot on that front and was basically just struggling to keep up without adding much value. Few web developers will mourn the need of having to deal with Edge specific rendering issues. Most users won't be able tell the difference. And having MS scrutinize what Google does with Chromium and kicking their ass in the standards bodies might help in keeping them honest.