This option is very useful if TinyMCE is used in a DHTML tab system or other divs that are hidden and displayed by some JavaScript. Since Mozilla has a bug when using display:none, the designMode state gets lost. To prevent this from happening you should enable this option. The default value of this option is "true".
Example of usage of the auto_reset_designmode option:
tinyMCE.init({ ... auto_reset_designmode : true });