I can't see the preview of my questions or answers in Waterfox/Firefox while I'm writing them. I didn't change any settings, it started just a couple of days ago.
I tried deleting cookies but it didn't help.
Any suggestions on how to fix it?
WHile it's not happening to me, a couple of days ago is when Firefox 11 came out. So it's possible the upgrade for you has broken something. You could perhaps try the Beta or Aurora streams? Alternatively a full reinstall of it, perhaps.
Also check on meta.stackexchange - there may be others experiencing the same thing!