Hi Jazz,
Thank you thaidatelink..Btw your English is so good
Thanks for your compliment 
If you think adjustment to the slide speed would solve the 'jerking' problem in Firefox, you could adjust the speed in your animatedcollapse.js on line (est.) 62.
The default number '500' controls the speed.
Code:
$divref.animate(animateSetting, $divref.attr('speed')? parseInt($divref.attr('speed')) : 500)
By the way, if adjusting the speed doesn't solve the problem, maybe you should reconsider using the 'Jump to Top' function with an 'Anchor Tag' to jump to the top of the page with the function attached to the element which you initiate to expand-collapse the div.
I have tried the 'Jump to Top' function and that did solve the 'jerking' problem in Firefox.
With better luck, you should be able to find a free animated 'Scroll to Top' script which have a slicker effect
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