Same thing is happening to me! I’m also using Firefox 3.6.22 and NextGEN Gallery 1.8.3. I was using 1.7.4 before and had the same problem, reason why I upgraded thinking it was a version problem.
Now the weirdest thing is that I have another website that also uses NexGEN Gallery version 1.7.4 and the edit thumbs feature works fine in Firefox 3.6.22. I had both websites WP dashboard open side-by-side with the same NexGEN gallery version open, one worked and the other didn’t! Go figure!
I’ve switched to Safari temporarily and it works perfectly fine there. I suggest you do the same or try another browser, besides IE, of course! 🙂
