Firefox will not install

[BillW]

Firefox working until few days ago, then would not re-open after
closing (error message FF already running), It wasn't running, nothing
in Task manager, no processes running. Needed reboot, to open FF.
Okay, 95% functionality.
I've run a full Kaspersky AV scan, and system restore (twice).
Attempting to resolve the issue, I uninstalled, after copying files
from the default profile to a temp file for replacement. Wouldn't
uninstall from control panel without a reboot first. After
reinstalling, still wouldn't open. Tried replacing bookmark, password
etc files from previous installation, without result. On last attempt
at installation, setup.exe did not write a default profile folder
(even an empty one). I tried inventing one, without success.

I believe a lot of people are currently having similar problems, and I
would be very grateful to learn of a solution.