Firefox is actually coping with it quite well now, I can't get the lag I had previously, but I have recently rebooted and not opened the inspector console yet - I am sure that slowly kills my firefox (I miss firebug!).
Thanks for the link, I thought the problem was caused by the sheer number of animations I was running (12 x infinite), I see now it all depends on what you are animating and how.
The stop at 8% is because there are 6 slides all running the same 36 second animation, each with a different delay. They are all opacity 0 for 30 seconds of it, and have just 6 seconds of display each.
Do you have any advice on achieving that effect (I do like the fixed position!) without breaking firefox & without looking clunky on IE? I was going to use a slick slider instead, and write some animations for the slides, but if I make that position fixed I guess I'll have the same problems?
How is it that http://www.menucool.com/zoom-slider looks fine in edge? Much smoother than it does in firefox?