When the local storage space used by Flash is full, Flash displays a dialog asking the user permission to increase the available space. The problem is that the editor freezes and its not possible to do anything but restart the browser.
So to avoid this, the next time you start the editor, don't open the project, right click on the editor window and choose “Settings…” in the Flash menu.
In the Flash settings set the storage space to unlimited or 1 or megabytes.