Hi Rintu,
Thank you so much for your kind words and your feedback: it’s great you’ve been finding the content helpful.
As for the issue you point out, it’s got to do with how Firefox handles animations: when the browser window is not active, Firefox stops the animation, which is good for performance, not so good for this demo.
Because the carousel script relies on the animationend event to remove and then add the relevant classes, if you leave the browser window where the carousel is running before the animation has completed, Firefox stops the animation and the script can’t catch the animationend event. You’ll find an alternative solution by Jason Johnson and one by me in the comments above.
I hope this solves the problem for you 
Cheers!