Same issue, running Comodo, Ccleaner, and Firefox. I just got Comodo two days ago and started getting that warning when I run CCleaner. I said no the first few times, then I tried yes to see what would happen. So far as I can tell, nothing happened. Looks to me like you must say yes in order to clean your history and downloads history. The Cleaner's progress bar doesn't move until it gets permission from Comodo. BTW: in Firefox, "compact databases" is already unchecked.