Comment 12 for bug 221265

There are three additional preferences that when set and locked disable those buttons.

It's not tied to the locking of the homepage:

http://mxr.mozilla.org/mozilla-central/source/browser/components/preferences/main.xul#76 pref.browser.homepage.disable_button.current_page
pref.browser.homepage.disable_button.bookmark_page
pref.browser.homepage.disable_button.restore_default

If you set these three preferences to true and then lock the preferences, they will be disabled.

If you want to prevent the changes of the proxy preferences, you need to lock

network.proxy.type as well.

I'm going to mark this bug as INVALID.