Disable "Additional plugins..." alert in Firefox

My NoFlash experiment is going okay, except for the fact that Firefox (yes, I still use it) likes to remind me that I should install the Flash plugin. I disabled the nag-ware by:

1. Typing 'about:config' in the address bar
2. Accepted the warning message
3. Filtered for 'plugin.default_plugin_disabled'
4. Set the value to 'false'

Done! No more annoying prompts.