Firefox browser under Linux Mint. Both are the most up to date releases and have all fixes applied. The hardware, probably irrelevant, is an Apple MacBook Pro (and older one).
Update: It worked still on Windows 7, so I tried re-booting. That solved the problem. It looks like it had something to do with a recent Firefox update (Linux supporting software) as Firefox went through its "Checking plugins..." spiel when it was launched. For whatever it is worth, just quitting and re-launching Firefox did not work; it took a reboot.
PROBLEM RESOLVED
This post has been edited by JRG: 2013-April-05, 08:40