Uninstalling add-ons are too easy. What can be done to make it harder to do that?
A plugin to block inappropriate sites can easily be removed going to the add-ons manager page. Can there be a password set before reaching that page?
Alla svar (3)
No, that isn't possible. It might be made possible with an add-on, but I haven't seen one like that.
You might want to contact the author of that add-on and request that he add a feature to that extension so it doesn't even appear in the Add-on Manager, unless Firefox is opened in the Firefox SafeMode. RichieB2B did that with the Disable Private Browsing extensions that he just created. DPB doesn't even appear in the Add-ons Manager tab in the "normal" mode.
https://addons.mozilla.org/en-US/firefox/addon/disable-private-browsing/
There are ways to block web sites outside Firefox, that serve to block web sites for all browsers on that PC, including "portable" browsers in a flash stick. One way is by using a parental control program, another is by using the HOSTS file that is a standard part of the operating system.
http://www.internetinspiration.co.uk/hosts.htm
http://en.wikipedia.org/wiki/Hosts_file
You can hide the extension in the extension manager with code in userChrome.css to make it more difficult to remove or disable the extension.
Open the XPI (ZIP) archive and look in the file install.rdf to see the ID (<em:id>xxx</em:id>) of that extension, GUID between curly brackets {} or an email address if that is used as ID.
Add code to userChrome.css below the @namespace line.
You can use the ChromEdit Plus or Stylish extension to have easier access to the customization files.
- ChromEdit Plus: http://webdesigns.ms11.net/chromeditp.html
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */
#addons-page richlistitem[value="<-extension_id->"] {display:none!important;}
Or you can use this add-on:
It can Hide any Menu's Entries :-P