Well, if you don't attach the Firepad instance to the element and it's in a closure I'm pretty sure you couldn't modify it. I'd love to be corrected if this is incorrect.
Obviously you can't be stopped from editing the contents of the editing area, so if it works by simply checking for your updates to the editor then you can't stop it.
Browsers have debuggers, which allow users to set breakpoints and inject/run arbitrary code in the context of some function. (At least Chrome allows that, not sure about others.)
Obviously you can't be stopped from editing the contents of the editing area, so if it works by simply checking for your updates to the editor then you can't stop it.