Go to Solution.
Start Firefox in Troubleshoot Firefox issues using Safe Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).
Can you close that box with the ESC key?
Don't you see an OK button?
The ESC key does close the box. I can see the OK button but it doesn't do anything. I noticed this weekend that this problem only seems to occur on a 32 bits Windows XP or Windows 7 and not on 64 bits versions of Windows
It was indeed caused by an custom Add-on. I notified the developer of this add-on.
Thanks very much.