Same here. Worked perfectly for a few days, then today, for no apparent reason, I got that "No internet connection" error. Tried to reinstall, can't login at all now, "Unknown error", it says. You finally released a real clipper for Firefox (and I guess that delay was Firefox fault), but it does seem to be full of bugs like these.
I don't know if you hate Firefox or if it's hard to work with, but I would really appreciate someone looking into this, especially since a bunch of people has the same problem. I've seen a lot of threads on this forum and others over a long period of time. And no, using the old version of the Evernote Clipper isn't a solution (if that works, like the post above me claims). That Clipper was the whole reason I was forced to use Chrome for several years because it didn't have enough functionality.
Edit: After 3-4 reinstalls and restarts of Firefox, I was finally able to login and clip again. I can't tell you why, but clearly, something isn't right, so most of my post still stands in my mind.