I have noticed an issue with my gallery thumbs overlapping when you resize the browser window, this issue is not present in Chrome as the images auto size to take up 20% of the window, but in Firefox 13.01 & IE 9.08 the problem is visible.
I have played around with the nggallery.css file, and returned this to the original to see if this was the issue, but it had no effect.
If you follow the link to the my site and load this gallery up into Chrome and one of the other browsers, then play around with the browser size you will see the problem first hand.
Not necessarily after a solution but some ideas of where to look would be great.
Thanks in advance,