Focusing on the mainstream user at this point will not help as they're already so far gone they don't even remember the name Firefox anymore.
They should focus first on making it and excellent browser for the users that still care. Then word of mouth will bring it back to the mainstream as it did the first time.
It's more that they're removing not-Chromey bits, not adding the Chromey bits, and removing Firefoxy bits like search keywords via bookmarks and the like.
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.
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.