Under Linux the option System -Cache will clean Firefox cache because Firefox is storing the cache in the universal cache directory ~/.cache/mozilla instead of under its own ~/.mozilla directory.
To exclude Firefox cache, either
1. uncheck System - Cache or
2. open the BleachBit preferences and add ~./cache/mozilla to the white list
Under Linux the option System -Cache will clean Firefox cache because Firefox is storing the cache in the universal cache directory ~/.cache/mozilla instead of under its own ~/.mozilla directory.
To exclude Firefox cache, either
1. uncheck System - Cache or
2. open the BleachBit preferences and add ~./cache/mozilla to the white list