I had this show up repeatedly while making updates at Bank of America Bill-Pay data: "A script in the extension "True KeyTM by McAfee" is causing Firefox to slow down."
And when I tried either "Temporarily disable extension on page" or "Stop it" neither seemed to work, as the problem repeated itself. Only by disabling True Key did the message stop showing up.
I am using Firefox version 84.0.1 (64-bit) True Key version 4.3.0.9324. Screenshot attached.
Rather frustrating, especially when disabling and stopping didn't work.