It's a bit of a dog whistle, I just don't entirely understand for what yet. Basically you're better off not asking and going on with your life.
A charitable answer is, however, that a central source of income for Mozilla is Google paying them to remain their default search engine. Mozilla is hesitant to truly attack Google, as it would be biting the hand that feeds it.
More importantly though, Mozilla has a female chairwoman. A lot of tech savvy people would rather stick with Brave, whose CEO they can relate to.
I find it hard to believe anyone can have such an incredibly clairvoyant understanding of the tech industry that they manage to see Mozilla as an evil megacorporation, yet at the same time failing to see any fundamental problem with Brave.
It could be a lot of things going on other than just sexism, but I cannot help but feel like any time a woman takes the lead in an open source organization a bunch of often vague but always hateful discourse follows in open source forums. Most people are of course fine, but a toxic minority will usually manage to get some weird discourse going that spreads to anyone taking whatever they spew on face value.
Often it can be hard to distinguish valid criticism from less than valid criticism, and in the case of big organizations there is always valid critiques to be made, so I don't blame people all that much for falling for it. Still, being a happy user of both GNOME and Mozilla products for more than a decade, it tickles me just how much hatred these projects receive online.
That's my five cents anyway.