Her mouth.
And she is also the daughter of James Comey. Which is 100% the real reason she was fired.
Not that that's better, Trump only learned she was a prosecutor because he was looking at the Epstein files that he claims don't exist.
It's important to understand Trump's thought process here. Trump holds grudges for decades, and he's still mad at Comey for the whole investigating Trump's ties with Russia and their blatant intervention in the 2016 election.
He also told his followers to ignore Epstein, and he sort of expected them all to obey. Because they have before.
So yeah, he didn't think anyone would connect this firing to the Epstein files, even though those files are the only reason why this firing happened, because Trump would have never seen the name Comey again otherwise.