It is easy enough to remove that, but then you also need to remove the Task manager and the run option. And of course remove access to IE. And also the Windows key +R option.

Does the extensions website offer any advice ?

Also what is to stop the teenager simply using Tools -> Add-ons from the firefox menu, does that allow it to be disabled ?

I suspect you may need to look at another method.

1. Right-click that Start Menu item for Firefox (SafeMode) and Delete' that menu item.

2. Visit a Windows support forum to find out how to block or hide the Run.. box from appearing. Typing firefox -safe-mode will launch the Firefox SafeMode - so you need to block the Run box from appearing in non-admin accounts.

Or, use a piece of Parental Control software like K-9 that works for all installed browsers, plus those that might be "plugged in" via a USB key. All too easy to use Portable Firefox on a USB key and with a desktop computer a "parent over shoulder" might never notice that Firefox Portable was being used instead of the version installed on that PC.

http://www1.k9webprotection.com/

Another option you may consider exploring:

The Safe Mode feature can also be disabled by modifying firefox files, that is explained in answer to How to *permanently* disable Firefox Safe Mode option?

Remember to password protect all admin accounts, including the normally hidden System Administrator account (which probably has no password set by default), but make sure you have that passwords secure somewhere, &/or have a password reset floppy.

A determined & knowledgeable teenager will get past most things you attempt to do, especially if you are not actually watching the computer use; maybe even running a different OS from a CD. Quite possibly the teenager has unrestricted access to the internet elsewhere anyway.

Thanks. I had tried to right-click delete from his user menu and couldn't. DUH...had to do it from my admin screen. That will slow him down while I look at the other suggestions.