I've had this happen a couple of times, too, but not all the time. Sometimes I can refresh the page and it rights itself. At other times, I have to log out and log back in to get it to work. However, sometimes neither "fix" fixes it.
Sometimes I have part of the text running off the right side when I preview a post and can't read the whole thing with FF. There have been a few times when reading posts, part of the text is off the right side and I couldn't scroll to get it onscreen. I think those times I had enlarged things more than usual (CTRL +) to read more easily. Considering all the variables they're dealing with, different browsers and operating systems, I'm amazed there aren't more issues.