An old article on this is https://support.mozilla.org/en-US/kb/Firefox%20cache%20file%20was%20infected%20with%20a%20virus though the steps in Settings have changed https://support.mozilla.org/en-US/kb/how-clear-firefox-cache You are not infected if this was only found in the Cache folder as it is harmless there if left alone there. It will either get overwritten over time as Cache gets used or deleted if you clear the Cache.