How to prevent a Firefox parental add-on from being bypassed by editing files in the profile directory or creating a new profile?

Question

FoxFilter is an add-on for Firefox that is supposed to provide parental control.

However, I found the add-on to be easily bypassable in at least three ways:

1. By running the Firefox profile manager and then creating a new profile. FoxFilter is a profile-specific add-on, so the new profile will be unaffected by it.
2. By navigating to the profile directory (in my case it’s ~/.mozilla/firefox/mwad0hks.default/ ), then to the "extensions" folder, and removing the "[e-mail]" file.
3. By starting Firefox in safe mode, firefox -safe-mode

Is there any sane way to prevent this?

One way I was thinking about was to adjust the permissions of files in the ~/.mozilla/firefox folder. To be more exact, I was thinking about prohibiting editing any files in ~/.mozilla/firefox, and in ~/.mozila/firefox/mwad0hks.default/extensions. This will prevent the add on file from being removed, as well as prevent the Profile Manager from creating a new profile.

It seems that by doing so I didn’t (yet) break anything really badly, since I’m still able to write this post. But still, is this enough? Are there not any other files that need to be taken care of to secure things out? More, are there any things that are likely to get broken badly from doing so? For example, I am now unable to install other add-ons. This itself isn’t anything too serious yet, but I’m wondering if I’m not breaking things silently but badly in the background…