I agree, it's untrue. In my opinion, most of the problems Firefox has stem from mismanagement and utter lack of vision.
This has a lot to do with mozilla losing their leaders and rock star devs at a rapid pace in the last 10 years. Some golden-parachuted out to the FAANGs, others just shrugged seeing the Mitchell Baker Club got more and more influential in a bad way and looked for less toxic places to work, and of course Eich prominently got cancelled for being against gay marriage and his departure rippled through the entire community. I was subscribed to planet.mozilla and there was a constant stream of "it's been fun, I am moving on" blog posts by names I immediately recognized.
The Code of Conduct situation caused a minor exodus especially in the volunteer community, especially outside of the US. A lot of rather important people, who were the ones building the local communities, just had enough with mozilla corp leadership unilaterally pushing stuff on them without soliciting any feedback first. mozilla was supposed to be this community of equals and Corp dictating more and more things really did not sit well with a lot of folks. Some outright (and sometimes rather publicly) left, others just dialed back their volunteer time a lot. This left especially the Western European communities in shambles.
The WebExtensions switch caused a lot of extension developers (who often were also volunteering in different areas of mozilla) to move on.
The loss of extensions, themes and customization options made a lot of the power users very unhappy. And those power users were exactly the kind of people who kept telling friends and family to use Firefox, driving Firefox adoption. While they often kept using Firefox themselves, they also quite often stopped advocating for Firefox in their circles.
All of this put together, mozilla losing leadership, volunteer, power user and dev mindshare at such a rapid pace then translated directly into a loss of market share.
Major failures such as FirefoxOS and BrowserID furthermore have been very demoralizing, making mozilla leaders very cautious, to the point where you had very little innovation going on from the top down. These failed projects - especially FirefoxOS - furthermore took away a lot of developers from the core product, leaving Firefox in a place where for years they had to play catch up with Chrome.
Things like Rust and servo where went more of a bottom-up direction, with bright engineers pushing it, not so much the leadership. And then these things were the first ones on the cutting block last year.
Google only "helped" in so far that they kept mozilla busy playing catch up, making the situation worse by a ton of (experimental) features and new specs that Firefox then had to implement, thus taking developers from other areas that needed improvement. I think most of the time it wasn't Google's intention to fuck with mozilla tho, it just happened to be the outcome.