Ad hominem applies to arguments. The source of an argument does not affect the soundness of that argument.
But it's not a fallacy to question an overarching narrative based on the source. If a person keeps selectively choosing facts and twisting words to forward a specific narrative, it's not fallacious to view what that person says with skepticism.
And other people are doing that in the comments. I addressed your point about ad-hominem specifically. So your response is kinda irrelevant to what I wrote.
People are questioning the narrative the author is painting based on their motivations. That's different to ad-hominem.
It absolutely matters. We need to consider that a right-wing actor is likely to exaggerate claims against an organization that is ostensibly socially-minded and represents anti-corporate interests, like Mozilla.
I’m not denying that Mozilla has a history of poor governance. But they are the competitor to Google here. You need to consider these things in context to understand what anti-corporate means for the internet.
The author clearly has an issue with the money going to left-wing orgs specifically. They're making a big point out of all the antiracism and one of their bullet points asks why Mozilla has no problem alienating their user base.