To fix, remove your site from the list of sites Google Gears is enabled. To do this from Firefox, click the “Tools” pull down menu and select “Gears Settings”. Click ‘remove’ for the sites listed that are having the problem.
The only way I could see WordPress developers from preventing this from happening in the future is to rename the wp-tinymce.php file for every release. There may be some documentation for Google Gears as well to tell it to upgrade all of the locally cached files.