Firefox pages are indeed protected pages, along with some other Mozilla-operated websites. This means that add-ons can't modify them like they can on other pages.

You would hide this element using a userContent.css file (which is similar to a userChrome.css except that it modifies the content of pages instead of the Firefox interface.

In your Firefox profile folder, you will need to go to the chrome folder (you make need to make it) and create a file called userContent.css with the following code in it:

@-moz-document url("about:home") {  
     #contentSearchSettingsButton {
          display: none !important;
     }
}

Once you save the file and restart Firefox, you should see the change.

Hope this helps.

Wesley, That's perfect, does what I wanted. Thanks a lot.

Actually, scrub that, it works on the home page but not on any new tab pages. FF treats a new tab as different to home page even though they are both set as Firefox Home (default). I can't see an equivalent to about:home for a new tab.

Oh, you're right. Firefox does appear to treat the home page and new tab page differently. In that case, change the CSS code to include the new tab:

@-moz-document url("about:home"), url("about:newtab") {  
     #contentSearchSettingsButton {
          display: none !important;
     }
}

That should work for you.

That's it. thanks.

No problem. Glad I could help.