I imagine, as with all Google products, it wasn't a suitable data point for them and didn't feed in some way to their ADs cash cow. Google nukes things that don't feed their ADs beast.
> I made sure to check all the headers in the original message, and everything looked correct
Headers are a bad heuristic for discerning sketchy emails I've found, since they're a garbled mess & hard to make sense of without some specialized tool, and clearly Gmail doesn't understand the headers either and had a false positive.
I imagine because the from address being `[email protected]` probably raised some flags since there's obviously some ongoing phishing campaign to takeover Firefox accounts, that you're unaware of, until now.