Here's a weird one, on two different installs I ofton can not get a cursor in the body area of new messages or replied-to messages. Also when replying the original message is not included even though that option is turned on.
On occasion I can see the replied to message for just a second as the rich html editor is loading and then it disappears. Also on new and replied-to messages, if I change the font options the cursor will show up and I'am able to type again so I think the editor may be the culprit.
Turning off sending html messages fixes the problem. This sounds a little like this issue
This is happening on firefox on linux and windows, clearing the cache has not helped either.