Mozilla has added small advertisements to the new tab page of Firefox’s roughest-edged build, the channel dubbed “Nightly” because it is updated each evening.

The ads, which Mozilla’s calls “sponsored tiles,” were first discussed by Mozilla in February, but the initiative was criticized by Firefox users. The company, however, defended the in-browser ad project, saying it was important to find other revenue sources besides its long-standing deals with search providers like Google. In May, Mozilla reiterated support for the ads but also promised users that Firefox would not become “a mess of logos.”

Mozilla has switched on the sponsored tiles in Firefox Nightly, which is analogous to a pre-alpha edition, and intended primarily for developers and code contributors. Mozilla maintains three more-stable build channels — Aurora, essentially an alpha; Beta; and Release, which is the final production-quality version — but Nightly is where it debuts changes and new features.

The Next Web reported on Firefox Nightly’s new tab page advertisements on Thursday.

The concept is straight-forward: When new users start Firefox, they will see pre-populated tiles, some of them “sponsored” — in effect advertisements — on the new tab page. For long-time Firefox users, that page, which has room for nine thumbnails, shows the most-frequently-visited websites. Someone new to Firefox, of course, would see nothing. To jump-start the experience, Mozilla will fill the spots with a mix of sponsored and unsponsored tiles. The latter will lead to popular Internet destinations such as Facebook and YouTube.

Current users of Firefox Nightly, who have their tiles already filled with sites they often visit, can see the sponsored and unsponsored tiles by deleting the existing ones on their new tab page.

Computerworld, which used an updated copy of Firefox Nightly tied to an existing Firefox profile, did just that, and confirmed that the new tab page displayed sponsored tiles. Those tiles led to BBC.com, Booking.com and Wired.com. All three were marked as “SPONSORED” in a smaller font size below the tiles.

Non-sponsored tiles also appeared for Facebook; Webmaker.org, a Mozilla project to promote coding and creativity on the Web; Wikipedia; and YouTube.

Firefox Nightly is in version 34, which is slated to reach the Release channel on Nov. 25. However, Mozilla has not said when the ads will be inserted into the Aurora channel, and the sponsored tiles could appear in a final edition much later than Firefox 34, or never make it to Release.

In May, Jonathan Nightingale, vice president of Firefox, said that Mozilla would experiment with the ads, but did not name a timetable. “We’ll talk about what we learn before anything ships to our release users,” Nightingale promised.

Mozilla’s stated purpose for dropping ads in the new tab page — to diversify its sources of revenue — makes sense: In 2012, the last year for which the organization has released financial figures, revenue from its contract with Google accounted for 88% of the Mozilla Foundation’s $311 million income. Royalty payments from Mozilla’s multiple search deals — which makes various search engines the default in Firefox — represented 98% of 2012’s revenue.

The most lucrative of those deals, the one with Google, expires in November.

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