Update, post edited
So here's what seems to be happening:
I had set recurring daily tasks for certain daily habits that were to be scheduled in to each day. Apparently the ones that won't drag are the recurred (is that a word?) ones. So, the original task drags, but the recurring ones don't. This is true whether "create new task" is checked or not. I didn't catch it because they had been recurring for a few days before I went in to schedule them. Also, some were set to weekly, so they were starting on that day, and did drag. So the problem ones were the 'ghost' tasks that have the dotted border around them.
I'll review recurring tasks again because it just might be a case of me using them the wrong way. Hope this update helps.
Screenshot attached. Restarted the browser, and saw this.
Here's what was happening: I've been having drag and drop troubles in the calendar. I was trying to drag a scheduled list from "all day" to 2:00, to schedule it as a time block. (update: just learned this is not possible). Went in and rescheduled it manually for 2:00 (I think from the task in the calendar itself, but not completely sure if it was that or the list on the left in the sidebar), but it didn't appear in the time slot.
So I decided to restart the browser. Upon opening, this message appeared. It loaded when I clicked continue.
Browser is Waterfox classic on OSX 10.8.8.5. No browser issues with WF so far otherwise. Been working flawlessly. Hope all this detail helps you to reproduce the issue.