There are still a few sites that run on Internet Explorer only and QuickBooks Online is one of them. I don’t recommend it, by the way. It’s slow, clunky and often has connectivity issues. But if you’re going to use it, you might want to use it in Firefox rather than in IE itself.
If so, you’ll need this plug in for Firefox. It is for Windows only, as far as I can tell. Sad all around. It means that I still can’t use QBs Online on my preferred and well-loved Mac. But at least I’m not bound to Microsoft.