Good news, all. I last tried installing the latest beta version of ff web clipper, but the problem stayed. However, today I uninstalled that and reinstalled the previous version and this time it actually cleared the problem! The yellow warning box was gone so I tried to clip an item. It first loaded the unsaved item that was causing the block and then loaded the new clip from today. No idea why this suddenly worked, but I am relieved to have my evernote back in use.
I can no longer clip any new item using Firefox web clipper since a previous clip failed (either I changed my mind or closed the page during the clip or something like that). Since then, there is a persistent yellow box over the elephant icon and I cannot clip anything new because iEN endlessly tries to deal with the old unsaved note. I have tried uninstalling and reinstalling FF Web Clipper, but it is still there. It looks like I can never use the program again and I am very disappointed, as you can imagine. Can anyone help?