I received a warning this morning to the affect that Firefox was at risk of security breach by running Flash Player so, since I've been noticing system sluggishness, I downloaded the current version of MalwareBytes and ran a complete system scan. It was eyeopening. 1025 errors detected!

A commonality was: corrupted install.exe files. I've narrowed it down to something in FF. When I tried to reinstall MalwareBytes, FF blocked it from being installed. A visit to Majorgeeks.com immediately sorted it.

If you haven't run a system scan recently, I highly recommend doing so.