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.
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.
https://support.mozilla.org/en-US/questions/1274579