Thanks gazumped, you are right about cookies on safari. We used the Mac Cookie App, and we had to specifically state that don't delete the Evernote cookie.
But this had not been necessary until the update 18.104.22.168.
In the case of Firefox is even worse. Since the last update Clipper only opens a blank tab and nothing else. We have set privacy to clear cookies on exiting out and we had already seen a similar problem in an earlier version but it was quickly fixed with the next.
P.S. We don't have any Adblock, only Tab Mix Plus, Pinterest button and Pocket installed.