This bug became present in v3.11.1541 and carried over to v3.11.1550. It is not present in v3.10.1525. Running CCleaner thinks that Firefox is still open. It is not. This bug still occurs even with a fresh boot, when Firefox has never been opened yet. Attached below is a picture of the error message. Please fix this bug as I'm forced to use v3.10.1525. Thank you!