Please ensure you have Adobe Flash Player 25 PPAPI (alongside NPAPI) installed on your computer.
Websites typically throw such errors when you have not installed PPAPI Flash Player, or the version you have is severely outdated. Opera and Firefox rely on different versions of Flash Player. You may download the PPAPI version of Flash Player from here.
As with everything, keeping software up to date is typically the best thing to do.
Your arugment is flawed because session cookies are still valid in memory for visited websites. There is no reason the tab cannot be reopened from memory. It only matters that they are not saved to disk and recoverable once the browser is closed.