"keylogger" may not be the right term here? I'm not familiar with how that term is broadly used for, but my definition of that term is a tool that logs your keypresses. Here, it seems like it was scraping your chrome/firefox data for login cookies?

Honestly there's quite a lot of malware that go against those files, I wonder if there's a way to require high privilege to accessing chrome/firefox appdata, or just block it entirely from other apps.