Original work (beware: its too big and its 2 pictures
Preview/Save
Well, so, Firefox gives you the option to choose between two different posting modes. You can switch between them by clicking on a box located above the "Smilies" thing and to the right of the row with all of the BBCodes (such as [CODE], [SPOILER] and [/JO]). I can't post a screenshot because I'm using my phone.
Anyways, one of the modes (the one you are using now, which makes the post box grey) can screw your post and make things like that happen, since you don't really type the codes and everything is pretty "automatic", thus making the post a mess. Try using the other mode, the one which the post box is white and see if the error happens again.