Just interpreting what the OP said and not really sure where I stand on the topic, but I think he means that "social justice" controversies have purged asshole devs that were competent developers producing good features and that firefox as a product is not as good due to that. Taking senior engineers off the roster will usually impact the product whether or not they were assholes.
As far as I can tell, OP's talking about the replacement of Mozilla Corporation CEO Brendan Eich, who had an engineering background[0] and was CTO but made a small but controversial donation to an anti gay marriage campaign, with Mitchell Barker, who came from a legal background and was formerly the president of the Mozilla Foundation.
At the time this was pretty divisive, because it was a candidate with objectionable social opinions but great technical chops getting ousted by a candidate who was uncontroversial but seen as more business-minded and more inclined to broad-spectrum activism outside the confines of Firefox's typical free software and online freedom work.
[0] he was the creator of javascript, among other things
Your timeline is off, FWIW. Brendan Eich resigned in 2104 and was replaced by Chris Beard as CEO from 2014 through 2019. Mitchell Baker became CEO in 2020.
In my experience Firefox has become a much better browser in the last 18 months, so if there was an asshole purge then maybe that helped?
If Firefox is missing specific capabilities that make it noncompetitive, and those deficits are traced to the asshole purge, then @rpnx might strengthen their argument by citing those examples.
Yea, could be that a purge helped. I tend to think assholes produce a toxic working environment and are almost like malignant cancerous tumors in the way they impact a company in the long run.