Hello Ruhul Amin,
Great plugin! Works great for our site. I have one issue though that I am able to replicate on multiple machines though. It deals specifically with the firefox browser.
After adding an item to the cart when a user tries to update the quantity the cart clears. Also, when a user adds an item, then returns to the store and adds a second item, it goes to the shopping cart and the first item is gone (only the second item shows).
There are two of us working this issue in different locations and both of us are having the same issue.
The site is
Sessions are working perfectly (I was able to find one other thread where you appear to point that direction)