Are you running Firefox in Windows compatibility mode? There's a current bug for users running in Windows compatibility mode. It's a known bug (see bug 1614885) that will soon be getting patched.

If you uncheck the Run this program in compatibility mode for and all other compatibility options on the properties window for the firefox.exe program and then restart your computer, that might fix your issue.

For a more detailed explanation, I'd recommend looking at this post by jscher2000, since he does a good job of explaining how to perform the above steps.

Hope this helps.

Not unless Windows put it in that mode by default. Everything was fine before I ran Windows update.

Confirmed. It's not running in compatibility mode.

There was an update released today. Does installing it from the Mozilla website fix the problem for you?

That worked! Thank you!!

mike

No it did not after a week of this apparently no-one care about it