You would have to exclude the cookies to preserve data stored in a Flash local storage object.

Let cookies expire when Firefox is closed to make them session cookies.

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: Keep until: "I close Firefox"

Create a cookie 'allow' exception for cookies that you would like to keep.

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: Exceptions

Thanks,i tried that but still the progress is gone when i return after cleaning history.

Any other ideas on solving this? Thanks.

What history items are you clearing and how/where do you do this?

Just a clear from the browser

History>Clear Recent History

These are checked

-Browsing & Download History -Form & Search History -Cookies -Cache -Active Loging

If you include the cookies then you shouldn't be surprised that lose the current state of your games. You need to be aware of the effect of each item that you clear and just clear everything just because the is a box that you can put a tick in.

If you clear cookies that way then all cookies will be removed, including cookies with an allow exception. You would have to clear cookies manually in the Cookie Manager if you want to keep some or otherwise let cookies expire when you close Firefox. You can visit specific websites in a New Private Window to avoid storing any data on the hard drive, so there is no need to remove stored data. PB mode exists to make this easy. Allow and block exceptions work in PB mode the same way as in normal mode, you only have a separate cookie jar in PB mode that is kept in memory.

I just want to play the game and keep it's cookie but delete all others. So the only way around it is to either browse in PB or manually delete all cookies except the game one?

If it works if you keep the cookies from that website then you will have to make sure not to delete those cookies. Note that websites can also store data in a Flash locally stored object (LSO) or in DOM local storage.