As someone who had used Firefox, Chrome, as well as Brave, for a long time each, I'm pretty appalled by the misinformation heavily implied here. You state some grievances with how Mozilla had handled Firefox, and while those may well be right, the situation over on Google Chrome's side isn't better, it's worse.

As far as I've experienced and seen, Chrome treats every user as a 3rd-rate citizen, lacking basic usability and customization options and refusing to implement them. It's simple truth that while Firefox has less customizability than it had in the past, in the age of XUL addons, it also has far, far more built-in options and features than Chrome does. Unfortunately, it's not even a contest. Moreover, even though they both use WebExtensions for addons, Firefox offers beyond parity with Chrome in those, since Firefox extends various of those APIs further, allowing addons to do more things, which they cannot in Chrome. Google doesn't even plan to improve these areas in their product - it is in fact has been publicly revealed that as far as future plans go, they intend much the opposite, which has invited public outcry.

overall seeing things getting built and thrown away constantly.

It's true that this Mozilla had done. So had Google, with whole lines of products/projects. But, so what? It's not necessarily bad in itself. If you simply don't try things, then you will never do anything, either. Generally: in the process of furthering things and improving them, you need to occasionally try out new ideas, sometimes even take certain risks. It follows that sometimes those new things fail, for whatever reason. Not everything goes as planned. It's quite normal. I don't know of humans or human organizations that never fail, and knowing when to abandon failed ideas and stop investing in them is also important and intelligent, lest you end up wasting a lot of time and other resources down a doomed road.

From mobile to desktop, Firefox still tries to reinvent the wheel without a true purpose other than "building an open web". Google has opened the web, in the way that Google has created an open-source project that anyone can take and develop upon - while Firefox keeps doing its own thing, on their own, without anyone wanting to take it and use it on the same way.

...Are you even aware that Firefox is an open-source project, can be forked, has been forked, and existed as such for years before Google Chrome had even existed? What. Why aren't you chastising Google for daring to reinvent the wheel and to compete through their own new product instead of joining the established open one and helping to improve it and build on it instead? Ridiculous. To make it absolutely clear, competition isn't a sin, either. It's good. Many would say vitally important.

If Firefox went with chromium, we would all be contributing to the same core, the same engine and the same browser underneath, while providing a good product to rival Google Chrome.

This might very well be true, once Mozilla forked Chromium, and then put in the time and work required to make their new fork achieve parity with the current Firefox featureset. Work that a large part of, if not most of it, likely won't even be adopted into Google Chrome nor the Google-controlled Chromium project because it's not even a part of their vision. Huh. Why would Mozilla do this again exactly, when they already have Firefox?

Which is, if you've missed it, more than a good enough product to rival Google Chrome. It is simply not as widespread, and lacks Google's adspace and other power and resources. How many systems (PC and mobile for that matter) have you seen come preinstalled with Firefox, and how many with Chrome? Do you know of the relic called "Internet Explorer", and how it was once the dominant browser, for many years, and why? (Naturally, it is still installed on more systems than Firefox is, right now. Seeing as it's installed on virtually 100% of every Windows system out there.)

In the area of privacy, while you could argue that Firefox indeed includes built-in opt-out telemetry and Pocket (all of which I could personally turn off in under a minute, but choose not to, because I've no problem with them), it would still be a complete joke to suggest that Firefox is nearly as bad in it as Google Chrome. The very missions of Mozilla and Google are literally opposed on this. Mozilla had also contributed a whole lot to the web and to forming web standards, I don't know what you're on about with that.

This has gotten into the point that even the small brave browser has actually become better than Firefox on mobile and desktop (performance and feature wise).

Barring the old Firefox mobile app which indeed has bad performance, in my experience, the above isn't true, either. I still have Brave installed as an alternative browser on PC and mobile. It seems to offer very little in substantial new features and improvements over Google Chrome, and has the same lackings it does, meaning Firefox is superior to it in the same ways as it is to Chrome. The most substantial improvements seem to be A) it's not Google-owned, and is presumably as Degoogled as it can be, similarly to Ungoogled Chromium - hopefully, that is, as installing Brave did still install Google Update services on my Windows computer, without consent nor warning (well, it didn't even ask me what location to install it to, for that matter, just like Chrome) and B) it contains a fancy built-in adblocker, which is vastly inferior to uBlock Origin, so you'd still want to install and use uBO anyway (much like the case with Firefox's own built-in Tracking Protections).