Ahh... I'm so sorry... I can't get this to work in Firefox either... so it's a confirmed problem.
However, the new "core" that I have (haven't rolled it out yet because the forum upgrade is coming soon as well so I'm doing it at the same time) does have a confirmed fix for this...
There is a "button" to put a URL into which will put the picture inline as expected. I've just tested it on beta.vinocellar.com and it works fine with Firefox and IE.
Warning: if you go to beta.vinocellar.com please know that the skin isn't finished yet. I have a long way to go and many things (colors) are still in progress.