Strange one that I came across yesterday...
A user posted a LINK in tags, and it never displayed at all in Firefox.. not even when I went to 'edit post'.
But in IE8, a small img with a red X in it was showing.
Confusing really.
However.. back in Firefox when you click 'Edit post', click the little icon at the top right of the editor (Switch editor mode), and you can see the img tags and the link or whatever is posted.
Hope that helps.