It's unfortunate that after those changes, the Mozilla team did not take criticism in public forums very seriously and brushed it off as noise. I personally stopped using Firefox after some of those "architectural" changes started producing "dead" tabs - the tab could not be interacted after load.

In place of "We're listening", users that complained got a "Works fine on my machine" bundled with a few rude words in places like Reddit. Not the core teams fault, of course.

Someone with more time can probably set up a scientific test of the performance claim even today - set up a bunch of Selenium tests to open JS-heavy sites in multiple tabs on older versions of FF upto current version using BrowserStack or SauceLabs.

The question is, would the work have been done sooner, or more effectively if they had just fixed the C++ code rather than spending a lot of time creating rust, only to rewrite said code?

I mean to this day (and i just tossed off a firefox build yesterday to see if most of what I was saying is still true), the firefox maintainers can't even be bothered to fix the tens of thousands of warnings that appear when built certain ways. That is a really low bar to cross with a C++ project, and they haven't even bothered. I've worked on projects where we had a recommended compiler, but we always spent some time assuring that the project appeared to work with the latest gcc/icc/whatever, because at some point those newer compilers would become the defaults, and also because they frequently pointed out issues in the code. Its just a cheap way to fix undefined behavior bugs.

So, I think the only answer to that is yes, that long stretch before they dropped the rust code was wasted time/effort.