This is a known issue for Firefox extensions on Linux and Mac. You can see a dialog about the subject here: http://forums.mozillazine.org/viewtopic.php?t=1705925
If you press "Enter/Return" it should have the same effect as pressing the absent "OK" button.
I re-confimed that it works on Ubuntu, but someone reported that it wasn't working on Mac for the export keyword window.
This is a known issue for Firefox extensions on Linux and Mac. You can see a dialog about the subject here: http://forums.mozillazine.org/viewtopic.php?t=1705925 If you press "Enter/Return" it should have the same effect as pressing the absent "OK" button. I re-confimed that it works on Ubuntu, but someone reported that it wasn't working on Mac for the export keyword window.