Thanks for your detailed reply.
(1) As I mentioned in last night's note, I scheduled a Boot-level scan when I shut down. This morning, the scan showed these infections: Win 32:Medfos (Trj), Win 32: Ransom-LJ (Trj), Java:Downloader-GD (Trj), Win32-InstallCore-AM (PUP) and Win32:Evo-Gen (Susp). All were moved to the Chest, then Deleted.
(2) This morning, the Firefox warnings still appear.
(3) I did not see a MBAM log - just the three that I sent. How is this generated?
(4) Advanced System Care v. 5.2 (Free version) does not, as far as I can tell, include an antivirus component. It appears to scan and clean only. The last time I did a scan using it was back in April.
I do, however, have Windows Defender installed - I just checked, and this program deleted two Trojans on 7/23 during its scheduled daily scan - Win32:Karagang I and Win32:Siref.P Defender did not notify me of these deletions.
(5) Since I had already made changes to the system, I will await instructions from Essexboy and do what he suggests.
(6) There is one other computer in my office, using the same wireless router. We don't interact. It shows no behavior similar to what I've described. I will run a boot-level scan on this one.
I will avoid using this machine as much as possible until I hear from your specialist. If necessary I can run through the entire process again (sigh). Thanks again!