Yes, this works. For now. The bigger problem is that this will probably go away at some point and the "sponsored" items will have no way (or a very difficult way) of being removed.
This has happened before with the browser toolbar having an about:config option to disable enlarging behavior, now you have to do some hacky CSS thing.
Now you don't just have to anticipate having options removed; that's the explicit development plan. (Quote is about a different preference, but I think it makes the style of development clear.)
> We intend to add a keyboard shortcut for this[1]. In the meantime you can revert to the old printing interface by setting the `print.tab_modal.enabled` preference to `false` via the page `about:config`. In a few releases time, after we've finished integrating user feedback and polishing rough edges, we intend to remove that preference though.
they plan to remove a hidden about:config preference for the system print dialog and replace it with a _dedicated keyboard shortcut_. if you think that's evidence that they want to take away your control, i beg to differ.
No, to be honest, I misread and thought that the final sentence meant that they were taking all configurability, both preference and keyboard shortcut, away.
about:config
browser.newtabpage.activity-stream.showSponsored
browser.newtabpage.activity-stream.showSponsoredTopSites
there is also:
services.sync.prefs.sync.browser.newtabpage.activity-stream.showSponsored
services.sync.prefs.sync.browser.newtabpage.activity-stream.showSponsoredTopSites