I get the same problem. Until today, it was intermittent, but today, in a particular note, I have several "Untitled Attachment" files. This is going to rend Evernote worthless unless the issue is solved.
I think the problem is that Firefox now requires add-ons to be "signed" and apparently Evernote Webclipper (and LastPass, even more frustratingly) are not signed: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Distribution
Grrrr!!!
- Justin