I had something similar happen. I didn't get fired, but I worked at a small business, early 90's, first job out of high school. Someone acted like a Karen before the term was common. I told a co-worker the person had acted like a bitch. My first real job, co-worker thought they should talk to owner/boss about it. Got a stern talking-to about how we can't be referring to our customers as bitches or other such derogatory names. Yeah, okay, point taken, went back to my desk duly chastised.

Co-worker and I spoke later. She admitted why she'd told boss, but said boss busted a gut when she heard because, yeah, the customer was a bitch. But boss had to put up the appearance to teach me a lesson.