Tried it, got the same error after clicking the arrow once. Also, the bottom page bar is going through the bottom row of creatures. In fact, the "background" is completely off center, all of it on the right-hand side of the page. The left sidebar is all right, but the rest isn't:
After sampling other members' creatures at random, this seems to be the only user's creatures that are affected at this time. I got zero errors cruising through about a dozen other lairs.
OS: 32-bit Windows 10 Professional
Browser: Mozilla Firefox 51.0.1
Any other info you need, please feel free to let me know.