I'm not exactly sure, but I do know that Mozilla kinda fell off the bus when it came to implementing full HTML5 compatibility in their browser. It's not an issue you can fix, unfortunately.
I hope this isn't the level of performance to expect when runescape transitions to 'rs3', because the bestiary is extremely laggy for me and barely runable.
The visuals are nice, but the search function on the tip.it site means I will stuck with them. At this point you can't searching as easy at runescape as on tip.it
I hope that later on the game will be playable on older IE as well, otherwise I can't play it anymore from work.