Isn't your last paragraph part of the problem, though? To paraphrase, you use Firefox's password manager for things you don't care about. So, those simple passwords are tied to small accounts that, individually add up to nothing, but together start to build a little cache of your emails, throwaway passwords and other tiny bits of data that all get collated with other data scraped about you. This much larger data cache then gets sold and used I attacks like credential stuffing to access even more data, etc.
You're posture is assuming that if it doesn't matter to you, then it doesn't matter at all, and that simply is not true.
Surprising that someone care to invest so much effort in it unless it can unlock some institutional level threats to leverage on for some geo-political negotiation or at least plots between big companies. But impressive hack, not necessarily.
> little cache of your emails, throwaway passwords
I have five passwords in my Firefox manager. (More if I include the ones which are no longer valid, like a few ftp passwords, and passwords to routers I no longer use.)
I think I'm safe.
I avoid online services which require identity as much as I can, because yes, any data builds up. Which means, yes, I buy things in stores, not online, I use cash, not credit/debit/e-cash, and I don't use apps.
If you do use online services, apps, etc., then it sure feels like you are assuming that information leak doesn't matter to you, so it doesn't matter at all.
You're posture is assuming that if it doesn't matter to you, then it doesn't matter at all, and that simply is not true.