You can try editing your /home/user/.chroot file (install leafpad, open a terminal, and type "leafpad .chroot") then remove the # and change "#TMPSIZE=6M" to something much bigger, like "TMPSIZE=20M" (you have to look at the size of a downloaded flv file on your desktop to know what size you need).

In order for your new TMPSIZE to take effect, you need to make sure all Debian apps are closed, and then use the "Close Debian" icon on your menu. When you start Iceweasel again, it will use the new /tmp size. Also, see if there's anywhere in settings you can change the temp directory of that plugin to something other than /tmp.

A huge /tmp directory on the tablet isn't a good idea in general, so if you can find a way to change the temp dir in your plugin, that would be the best solution.