Well this isn't a huge problem, in fact I don't mind it much, I'm just wondering why this is happening:
Upon opening a new tab page, thumbnails usually show for your 9 "top sites". For me however, they usually show up, but on a particular website, it is not. I have no idea whether it has to do with Firefox or the website, but I figured it was worth asking it on here. (the website is http://thenforumsite.forumotion.com)
The thumbnails on the about:newtab page are generated from the cache, so if the page is not in the cache or otherwise can't or won't (secure pages) be rendered as a PNG image then you will get an empty box.
Pages that send a no-store or no-cache header aren't cached and thus Firefox does not generate a thumbnail.
hello ThunderLion, websites can specify that their content shouldn't be cached by browsers - in this case firefox won't create any thumbnails for them which would show up on the new tab page... this is also the case for the sample you've provided. this problem will likely be addressed in a later version of firefox.