Why is it a red flag if someone talks about getting fired?
It really depends if we are conflating "getting fired" with "laid off". The latter is not bad. When I hear that someone got fired it usually means they did something like break a big HR rule or caused the company to lose an important account.
Basically I found out the founder of the company had been lying to me as a early hire, and I asked about it and wanted it fixed. And suddenly performance reviews went extremely negative. Stuff like that is why people get fired.
Lesson Learned: Always get it written in a contact, don't trust verbal agreements. Companies only argue for "Professional" behaviour, when its in their favour.