Sorry, where are the ads? Do you mean the brief "snippets" that appear below the search box on the about:home page? Currently if I reload the page a few times, I see:

• Whether it's the first day of fall or spring where you live, the September Equinox means equal day and night while we shift seasons. Enjoy it!
• With Firefox synced to your Android, you can access your history, passwords and more from your computer. Install Firefox on your Android..
• Mozilla principle #8: Transparent community-based processes promote participation, accountability and trust. Learn more in the Mozilla Manifesto.

I don't think those are paid ads.

The Yahoo results page is controlled by Yahoo, so you can contact them about their ad rates.

The new tab page has some sponsored tiles. More info: New Tab page in Firefox

If you see a different page, can you identify the website or add-on generating it?

Hi:

I actually mean the native posts in 'favorite places' or 'recent visits'. When i load the page, there's an ad in the thumbnails of pages i've been to recently.

Jaime

This article has information about sponsored or suggested tiles on the new tab page. More info: New Tab page in Firefox. I don't know how partners are chosen for that program. You could try contacting the content services team: https://www.mozilla.org/about/partnerships/contentservices/