I'm having the same problem. I'm trying to copy and paste the overview text from a trip description at http://www.southernexplorations.com/amazon/amazon-machu-picchu into Evernote. I select the Overview in Firefox and copy it, but when I hit Paste in Evernote nothing goes in most of the time. Sometimes it works, but mostly not.
I can paste the same text into the vim text editor, and I can copy and past text from vim into an Evernote note, but something isn't right going from Firefox to Evernote.
And yes, I could just use the Web Clipper, but the workflow is easier for me to copy and paste, and copying and pasting is one of those fundamental things that is supposed to work.
Evernote 6.10
MacOS Sierra 10.12.1
Firefox 50.0