“Why fire Mueller if he has nothing to hide?”
Because generally in the history of mankind, you investigate a crime to find the perpetrator. You don’t investigate the person you want to be a perpetrator in search of a crime.
Another note, Mueller has ruined tons of lives in the past convicting innocent people. This is all well-known, common knowledge, not Alex Jones stuff. If somebody is targeted for his political beliefs by a prosecutor known to put innocent people behind bars it should be enough to scare you. The fact that they have decided to keep looking until they find (i.e. fabricate) something because they haven’t found anything on Trump yet should scare you.